Users repeating "thank you" phrases

When I first joined SG and saw people leaving thanks on every GA, I just thought that was the etiquette here. So I would copy-paste some generic "thank you so much for the GA" message on dozens of giveaways within the span of a couple minutes whenever I logged in to waste some points.

Looking back, it probably looked like bot behaviour judging by what you're saying, so it would be tricky to weed out which users are copy-pasting versus actual bots. I think an easier solution would just be to implement some sort of captcha for every x amount of giveaways you enter.

I heard that suspecting is not enough. You need something called "proofs".

It never happened to me till recently. Now it's happening to me more and more. And my giveaways are for level 2. I'm thinking of raising the level...

You could always just put "No need for thanks in the comment" or the more forward "Please don't comment unless you are the winner" etc.
People have done it before and the most users comply with the request.
Now if you put that in your GA and still get a thank you then you can start suspecting bots or just people who don't read.

Well, both bots and normal living humans post generic "thank you" comments. Sadly, it's not easy to distinguish between them, and I have been BLed by multiple people because of this.
I'd advice you ask not to thank you in the GA description. This way anyone who still thanks you is either a bot, or doesn't read description, which also is a negative point for them.

There's also chance ppl are using auto-comment on joining a GA, saw an extension few days ago here which allow to auto-post a comment when you join a giveaway.