What if the GA does not end yet and a person get banned by me while he is inside?

No, once an entry has been made for a giveaway that has not ended the winner can't be BL'd for that giveaway.

If you BL a person that has made an entry for your giveaway the BL will only work for your next giveaway.

Also I don't think that it's a valid ground for a reroll.

You can blacklist people that have entered one of your GAs but their entry will still stay valid. The blacklist only applies for future GAs. If they happen to win you will still have to give them their game unless they have broken the rules in which case you can ask for a reroll.

You can only ask for a reroll under certain conditions for example if they haven't activated previous wins, tried to regift or sell stuff they won or won the same game multiple times.

You could delete the whole giveaway :)

if it's your arch-enemy ?!

delete GA, recreate again, he cant join it xD.
but thats a harsh move to do