I'll probably comment a bit here about the OP's limitations.
Bitcointalk.org is a community based forum that any user can register for free in any country. People use these forums based on their interests and can rate each user around them in various ways. They should have earned the right to vote out of respect for the forum contributors all along, right?
Basically, what you stated above is true,
unless anything contradicts forum rules or OP's own rules (which also have to respect forum rules, of course). And here, the ones which do not have 50 merits and which do not have at least Full Member rank break OP's rules.
If they don't get 50 merit, maybe OP should also count them as valid votes because maybe they also have a clean heart to show respect to the users of this forum as I'm sure no spammer or cheater would dare to enter the meta for merit purposes.
The problem is not just that spammers may affect this contest. Their spam can be easily deleted, by having the posts reported. It's just that this is a contest and, similar to any other contests throrough the entire world, it has rules. Imagine at a football championship an amateur team comes in and states they want to be included too, although thei are addressing to the Professional League. It's simple: there are rules and they must be respected. Otherwise why were they set?
For not having any intention of respecting them? 
I wonder what you have said if this contest would have been held inside Ivory Tower board, where only Full Members and above can write. What would you have dine in that case? Report the entire contest to be moved to another board, for not allowing lower ranks to post in that board? Contest the rules of the board itself...?
Look, I totally understand your point here. And, normally, I agree with what you say, so don't take me wrong. But this applies only up to the point where rules are broken. From that point forward there is another discussion. You should understand that too...