I think that it is very difficult to prevent voting own posts by multi account. Although stupid spamer who makes accounts by same IP can block by IP, since It is easy to make accounts different IP, there is no method of proving that each accounts are same person. Moreover, although user who is doing only vote can be deduced by tracing specific user's action by DB, it is not bad to see only contents without posting. This is same as spectator of sports.
When it becomes clear that it is multiuser, in addition to the method of blocking by IP, there is method of giving minus-bonus-point(-999,999) to the spam user (aided by clone account).