An hypothetical for you...

I'd be polite and ask them to remove their entry. Does not work with a rule abuser, but nothing works with a rule abuser.

I simply would tell them to request a reroll if they should win. And deny giving the game, if they should refuse.

P.S: It's quite odd that you ask such a specific question, considering that you never even created a wl giveaway ,,,

I would rather be satisfied with less entries and haven't took CV, or if you need CV, recreating as public is option. You can even notice users regularly asking in 5-perpentual thread to be whitelisted for game they already have only with intension to enter your other giveaways.

I wouldn't have this problem in the first place, problem solved!

In that hypothetical scenario I wouldn't care, since I'd be looking to get more, not less entries. But I never used that threat before since every time I had less than 5 entries it was a group giveaway, so I assumed that asking there would be pointless.

Usually I wouldn't mind. If I did,, I would politely explain their mistake. I would always assume a mistake.

If he refuses, blacklisted. At least temporarily. That way he loses access to my list.

politely ask them to remove their entry

mistakes happen...e.g. after getting added to your whitelist all your other GAs appeared in their GA feed together with the one you wanted them to join. so they simply might have not noticed the others where your other whitelist GAs they werent supposed to join.
so just ask and hope it happened somehow like this...
if not youre still free to pursue one of the other options

If I want to give a key to a certain person, I don't do a giveaway, I just give the key directly to him.
And if I do a giveaway, I don't care who's the winner.
But a rule abuser may or may not end up on my blacklist, depending on my mood.

Considering the consequences, I assume you won The Beast inside? By sg rules giveaway creator can't force you to agree to reroll and can't reroll without it, but tbh that game isn't that good to ruin your reputation for it.

TBH I wouldn't have stipulated that they couldn't join other WL giveaways in the first place. Seems kinda dumb to be willing to add someone to one's whitelist if one doesn't want that user to freely enter the WL GAs one makes, people are gonna do as people do and be tempted to enter the shiny exclusive GA with low entries, why would you show someone a plate full of cookies if you're not willing to let them eat one?