In order of priority:
Flag the post for moderator attention.
Downvote the post. (As Glen_b helpfully notes in a comment, if you plan to do this, do it first, because if yours is the deciding vote to delete, you won't be able to downvote afterwards.)
If you have enough reputation, vote to delete the post.
If you have enough reputation, vote to close it.
If you have the privileges to edit posts and the post is particularly obnoxious, consider editing out the offensive portions in the meantime.
Some explanation:
re (1): Moderators respond to the flag by tracking any other "contributions" made by the same individual and dealing with them all at once if necessary.
re (2): When a new user's first few posts are extensively downvoted, that user is automatically blocked from posting more material.
re (3): Posts can be both closed and deleted. Deletion makes them invisible to most users and so is preferred.
re (5): It's best not to edit a post unless it's obnoxious, because retaining the original text makes it easier for moderators to review it.
When a mod is present (which we usually are 24/7 due to our worldwide distribution) then the response to your flag is often swift. On this site, moderators typically have responded within minutes, if not within seconds, to such material.