If I create a tournament, I don't invite players at first. I create a game with requirements and then the tournament is created.
But when I forward invites from that point, I see a long list with members, including members of which I surely know they don't meet the requirements.
Can this be changed?

Open seat prerequisites only apply to those browsing the Open Games / Open Tournaments tab. There's no restriction on who you can invite directly. This is by design, since inviting someone directly is a way to make an exception to the rules and let someone in who wouldn't be able to join it without an invite.