By the police and other institutions, I guess. I don't know s**t about politics, but once in a while someone gets arrested, so I guess that it works to some extent.
As for legitimately buying power, in Poland the election campaigns are publicly funded for example, which is silly and has other problems, but at least it prevents someone with a lot of money from buying them a seat. If it was up to me, I would forbid media campaigns altogether, because they are just bulls**ting people anyway.
The biggest corruption here is where public organizations do business with private companies, but there is no good solution to this, I think.