If you see her talking to a girl, you can visit her in the tavern in the evening and forbid her to pursue her.
That might not be working correctly, then.
Before the quest is finished it works like it should and she'll leave them alone.
But, after the quest is finished she mingles regardless of what your conversation settings were.
e.g; telling her to pursue Ella but nobody else she'd still bang Lana in bed.
I made another save just to be absolutely sure.
But in addition to that, characters like Princess Lana you can't have a conversation about at all
Edit: I'll attach a save of a run I did just now.
From start of a brand new game until the completion of her quest. Waited to start the second half of her quest until all girls were there and all were forbidden
It doesn't seem to make any difference.