What is a spell for preventing a chest burster?

The typical spell for dealing with implanted eggs is Remover Doença:

The spell also kills some hazards and parasites, including green slime and others.

An example of a creature very similar to a chest burster is the Lunarma:

Implant Eggs (Ex)

As a full-round action, a lunarma can lay 2d6 eggs in a helpless creature. The eggs hatch after 24 hours and each deals 1 point of Con damage per hour as they feed on the host. After 1d2 days of eating, the larvae crawl out and find a safe place to mature into adults. A remove disease spell rids a creature of all implanted eggs or larvae, or they can be removed one at a time by using the Heal skill to treat deadly wounds.

The Lunarma record also suggests another alternative: Using the Heal skill to Treat Deadly Wounds.

Treat Deadly Wounds is also how you remove an implanted bomb:

Removing an implanted bomb requires a Heal check to treat deadly wounds, followed by a dispel magic spell or Disable Device check to neutralize the bomb (DC = 11 + alchemist's caster level).

Naturally, your DM is free to say "but mine are different."

Raise Dead is a 5th level cleric spell which you'll probably want to raise the townsfolk after you put 'em down the first time. Make sure you preface castings with Purificar Comida e Bebida (an orison)

Remover Doença, as @AceCalhoon notes, is the correct spell for easily fixing this at any stage of development pre-death. Restoration Domain gets it as a level 2 spell, which means finding NPC access for non-soul-ripping extra access should be relatively simple.

Parasite Removal, as @AceCalhoon also mentions, is usually also accomplishable via surgery with the Curar habilidade.

Limpar is a supped-up version of Remover Doença as a level 5 spell. It isn't actually needed for anything but if Remover Doença doesn't work, you should still check if this works since your DM might be giving the alien infestation some power like "Remover Doença doesn't end this effect; it takes more powerful magic, like Limpar, to heal an infected creature". This is the sort of thing one encounters with curses and Remover maldição fairly regularly so it makes sense to check for it.

Banimento, if the aliens are Not of This World and count as creatures, is one of your best options, especially given the time you have to study the creatures and any substances (e.g. light, fire) they fear/hate/or are repulsed by. You can banish at least 22 HD of stuff per round.

Proteção contra o mal, if the aliens or their creators are evil (as long as they aren't true neutral there's an equivalent spell), will prevent you from being zombie-controlled or going crazy while you are under its effects, and grant a new save to stop being a zombie to any zombie you cast it on. Obviously the thing to do with this is to cast Hallow (which grants free Magic Circle against Evil, which grants free Proteção contra o mal) on your base (possibly several times, depending on size), tying a Luz do dia spell to the Hallow effect, if that in fact affects the aliens negatively. Hallow is instantâneo so it's almost impossible to remove its effects even if you wanted to (you'd need Quebrar o encantamento. Even Unhallow won't work once the Hallow is in place; you'll just wind up with both), though the Luz do dia effect, if you add it, will only last for 1 year. Hallow is a 4th level spell costing 1000 gp to cast plus extra for tying spells to it. Proteção contra o mal is a 1st level spell.

Delay Poison isn't really worth using, even if it is allowed to apply, but it does do what you asked, stopping the progression for 1 hour per level per casting. It's a 2nd level spell.

If your DM has played Elona it is probable that the chestburster/fantasy mashup was inspired by that game. Anything which cures pregnancy in that game is a decent experiment if you're desperate. As a summary, these methods are:

• poisoning oneself
• Drinking strong acids
• Morte
• Inducing vomiting

Obviously precautions should be taken against the side effects of these 'cures' before attempting them.

Na mesma linha, Boiling Blood may damage the parasite as well as/instead of the host.

Lastly, if you are employing the third party supplement Book of Beasts, Summon Minor Ally can solve this problem for you if you can get access to it. The Hydrus is a Tiny animal who is tb a chestburster, but a much faster acting and less deadly/unpleasant one. Provided you're willing to have two-foot long chestbursting clawed eel things crawl down your throat, they should kill the other parasite on their way out.

Mudança de avião will save all of the party. Unlike with teleport, discussed below, there is no chance of accidentally taking any chest bursters with you, choose the 'up to 8 creatures holding hands' option, and then the chest bursters can only come along if they are holding your hands. Downside? Takes 2 casts to get back to the material plane and you are 5-500 miles away from your starting point.

Teleport will save all of you except maybe the caster (not that your party has a sorceror, and anyone who accidentally read summoner as sorceror would be foolish).

It can only teleport creatures touched, so it cannot teleport the chest bursters inside the rest of the party.

The chest burster inside the caster may count as a touched willing creature, which would doom the sorceror.