I found a post that I consider merely spam.
What should I do? Should I down-vote the spam, remove the spam link, or post a question on the meta site to ask them to delete the spam?
When you see spam posted on a Stack Exchange site, you should click on the "flag" link, and select "it is spam" from the dialog that appears.
When 6 users flag a post as spam, the post is automatically deleted, and the IP used from the spammer account automatically blocked. This means the spammer will not even be able to create new accounts using the IP already used to spam.
Moderators will then destroy the spammer account.
Don't down-vote spam: The flag for spam automatically down-votes the spam each time the spam is flagged with the correct flag. Unlike ordinary answer down-votes, this automatic down-voting will never subtract a point from your reputation.
Don't edit the spam links from the post: This makes difficult to find spam posts because finding them would require to look at the previous revisions for all the edited posts.
If the post is an answer, and it has been effectively written to answer the question, moderators could decide to remove the link, and leave the answer. In the case the answer is suggesting a module, or a specific tool, and the user keeps suggesting the same module/tool for every similar answer, then it is probable that one or more answers are removed.