Are custom rules for picking a winner OK?

That's not how this works. If you blacklist someone after they enter your giveaway, their entry still remains valid and you still have to deliver the game. All it does is prevent them from entering your future giveaways. Besides, blacklisting depends entirely on your whims - you are allowed to blacklist people with no reason at all.

As for special rules, it used to be possible to set special rules in giveaways, but a) you had to ask support for permission first, and b) you could do it only in private giveaways.

Related: the winners are randomly chosen. Is it OK to ignore that and choose someone else?

Sure, but they won't count as the winner and it will be marked Not Received.

You can't choose winner, site automatically does, and I don't think its a good behaviour to blacklist somebody just for saying "thank you".

so many helpful answers, i'll try to add mine.
you can blacklist people for whatever reason. you can get ideas here :(.

the site picks the winner, and there are a few rules that allow asking for a new winner. they are defined by the site admins. you cannot make a custom rule in the GA, you can ask people nicely to do something you want/need.

Probably the most polite opening poster (OP) I've seen.

View attached image.

You can't set custom rules on giveaway. If someone entered a giveaway - he have a right to win.
But, you can set custom rules for blacklisting - it's completely up to you. So, you could blacklist someone who disobey, and he will be unable to enter any of your givaways in future, but if he won in the one he already entered - you have to give him a gift.

You can blacklist anyone you want to, it will not affect the said person if he already entered your giveaway. It will though prevent him from entering any more of your giveaways in future.