How to deal with cheating VKontakte polls?

Question

In contact with

Hello, maybe not quite on the topic of the resource, but still someone can share their experience. VKontakte public admins, how do you deal with poll cheating?
We conduct surveys every month, with a prize, and every time there are cheats, disputes and claims.
At first, our polls were open and posted on the wall, but given the large number of voters, we could not check everyone who voted.
Then we decided to move the polls to discussions, in such polls only community members can vote. But that didn't help either. Participants join the community in waves, vote in waves, and then also massively unsubscribe. There is no doubt that these are cheating (voting takes a long time, and such an influx always occurs on the last day, one-time). The voters themselves, of course, always deny it, and we have no evidence of cheating and how the votes were cheated.
Maybe someone knows how to detect cheating or how to prevent them?

Answer 1

Complicate the procedure as a whole
Repost the news
write a comment under another news
have more than X friends
give access to the profile
be in the group for at least X days
write in the group at least X comments longer than X characters
But in general, the drama around the contest itself is more useful than itself

Answer 2

Count the votes of only those who were in the group BEFORE the start of the poll.