There will be a point where you “lock in” a relationship. You’ll commit to your RO, and in turn, they swear the same to you. The conversation, the tokens used (if any), and when it happens all depend, of course, on the RO.
That’ll pretty much block all the other romances but, honestly, at that point, those romances would have been blocked anyway. The characters aren’t dumb, they can see where your affections lay, and while some might not care in the beginning, as things progress, none of them wants to meddle between two people in love.
If you’ve kissed Hadrian or Alessa in front of Lance, for example, will already have an impact on a potential future romance with him — it’s not as if he’ll suddenly forget.
And even after those “lock in” points, I’ll probably add the option to break up at key moments in the story. Or the ROs can break up with you if you do something that goes completely against what they stand for.
But, until then, I’m not going to lock choices. You can flirt with multiple people, you can try to keep it a secret or not care entirely. The only thing I’ll say is that there will always be consequences to your actions. That’s it.