[Doitsu]

IMHO, the easiest solution would be to check the language of posts. AFAIK, vBulletin can be customized with PHP libraries and since there are a couple of ready-made language detection PHP libraries, e.g. language-detection, it should be relatively easy to implement a filter that'll reject posts by new members that aren't in French, German, English or Spanish.

It should also be able to detect Korean characters with one of the following regular expressions:

[가-힣]+

p{Hangul}