I don't think such answers meet the criteria for spam. The criteria for spam is that the post:
Exists only to promote a product or service, does not disclose the author's affiliation.
Sure, one can argue that they promote the poster's pet conspiracy theory, but many other answers also promote ideas shared by the poster, so that on its own shouldn't qualify a post as spam.
Conspiracy theory answers are, however, usually:
- Unrelated to the question they are posted as answers to → is not an answer
- Poorly written → is potentially very low quality
- Nonsensical or incorrect from a subject matter expert perspective → should be downvoted
Those sound like good reasons to vote down, flag as not an answer or very low quality, and vote to delete.
If the answer actually is (bear with me!) relevant to the question, such as a moon conspiracy theory being posted in relationship to a question on the Apollo program, and explains why it is relevant (in practice, that is a very big if), then it makes more sense to treat it as simply a wrong answer, which generally means to simply vote down and if possible comment to explain why it is wrong.