Please flag such posts as spam, and downvote them if you wish to get them off the front page faster, but please do not edit them. (This is also the current advice given network-wide.)
Here is why:
First, six spam flags from the community is sufficient to delete a post. But for a community member to know that something is spam, they have to be able to see it. If the post is edited, then they have to view the revision history first, before they can be sure it was spam.
The same is true of moderators: while we can kill the post immediately, we still first have to take the extra step to see what the post was.
Second, the spam flags feed into a variety of new features Stack Exchange has implemented over the past year to control spam network-wide. As does the act of destroying the user account, which a moderator will come along and do after the spam has been deleted.
Finally, it's not necessary to worry about search engines. Even though they keep a close eye on our site and have posts indexed within minutes, they will also quickly remove anything that was posted and deleted shortly after. Plus, new users have "nofollow" added to their links, so spammers can't gain any search engine benefit from them for the short time they may be visible.
In short, there's no real need to edit a spam post, and because it causes some inconvenience to others who may want to help get rid of it, it's better not to.