Questions tagged spam are pretty much, by definition, about how to prevent spam. (I can't think of any questions that we would want here that wouldn't be.)
I suggest, then, that the following tags be made synonyms of and merged to spam:
- spam-prevention
- anti-spam (and its synonym antispam)
I suppose an argument could be made that spam-prevention should be the "master" tag, but that and anti-spam are obviously the same thing.