Accounts connected: Pprosto, zardas
Proof: I've switched my giveaway thread to self-moderated. Pprosto is on my blacklist. After deleting his post twice, he posted the same username ("prosto") from account zardas. I'll delete this post too, but it's archived.
Pprosto has literally never posted anything else than username "prosto". I've archived his post history.
Zardas too has never posted anything else than unqualifying spam in my giveaway.

By posting with two accounts on the same day this user also tried to cheat this giveaway.

Addition: despite deleting his messages about 5 or 6 times, and sending a private message, this person keeps posting in my thread. I guess a self-moderated thread doesn't have any option to stop this (other than deleting it again), and reporting to a moderator doesn't work if all the evidence has been deleted by me already.
I expect this will happen more often in the future, so I'd like to know how I can best handle this.

I estimate a total of around 2000 spam/activity-posts in my previous giveaway thread (25% of the total posts), that's why I switched to a self-moderated thread now. I hope the spammers will give up after they realize all posts get deleted. This is the first spammer who switched to a different username.