I think there are a few separate issues here, that perhaps should be handled differently depending on the behavior of the individual(s). I'll just focus on one that I've noticed (at least one other that I won't touch on is students, like Ryan's, asking homework problems under pseudonyms).
The surge I've noticed recently is primarily PilarLoof, HeloLobo, and Grid_y_Bill, who I suspect are all the same person (and others have expressed the same opinion), and have been posting questions more suitable for mathoverflow, often without any indication as to why they are relevant to CS, although a few of them seem to be. When asked why they chose cstheory.SE over MO, they gave essentially nonsensical responses. (BTW, I think we definitely should open the migration path to mathoverflow, because of these questions. It would make them much easier to identify and deal with...)
In this case, I think it would be appropriate for the moderators to 1) determine if these pseudonyms are all in fact the same individual, and if so 2) ask for a good reason they keep using pseudonyms to post off-topic questions, and if there is not a good reason then, 3) take some sort of action.
What sort of action? I could imagine several possibilities, though I don't know which of them are technically feasible with the current SE software:
a) Ban the user (by which I mean the actual person, not just the username - perhaps by IP address as suggested by Saeed in the comments, or some other mechanism?)
b) Ban the user (same as above) for a fixed period of time - e.g. 30 or 60 days
c) Put the user on "probation" (possibly for a fixed period of time). I imagine this would be something like: any of their questions must be approved by, say, 3 or 4 users with sufficient rep before the question appears on the site. This would be a pre-filtering approach to questions rather than the current post-filtering approach. If the user continued to abuse the site causing high rep users to spend lots of time rejecting off-topic questions, then more drastic action could be taken.
d) Other suggestions...?