If the post "looks like" a real answer but the subject experts know it's not, I favor deletion (and deleting/suspending the user). Though kudos to the clever spammer I suppose :P
If it is a valid, accurate answer, we should:
Delete or suspend the user, so the problem stops.
Edit the post so that the useful content remains.
In place of #2 I would also be OK with creating a new post with the despammed content, although that seems like a relatively pointless diversion.
There's no reason to completely eliminate a useful answer. Doing so wouldn't be any better at ending the problem.
As I noted in the comments on Wipqozn's answer, I don't think either case is likely to occur very often at all. My concern is with the idea that deleting useful content is OK, since that extends to many more posts than just this class. If we as a community don't allow that then I am happy.