Is there a way to quickly end unwanted romantic relationships?

MCCC, remove the relationship and start over.
it would be great if relationships can be forgotten like sims if you don't spend time with them instead of being stuck with that relationship bar forever there.

I third the MCCC option. Works every time and effectively. Including total forgetting if desired.

The modify relationship cheat did not work for me, does anyone know if this cheat still works? Has anyone used it successfully recently ?

@RoseRyder it was still working for me as of last night. I’ve had to use it extensively to restore my sim’s relationships after moving my game to a new save. You have to be very careful with typing it all in exactly. Modifyrelationship YourSimFirst YourSimLast TargetSimFirst TargetSimLast number LTR_friendship_main. Number = -100 to 100. Substitute LTR_Romance_main to cheat the romance bar. Sometimes hyphenated names and multiple names (Like Mary Sue) are a problem with this cheat.

Lock the unwanted stalker in a secret room (preferably in a basement) and take away the door. Okay, maybe that isn't the fastest way but it's the most satisfying, er so I'm told... ?

I've had this happen in my game once autonomously... But aliens have a social they can use to "make a sim forget about them". And I think same for most of the other occults... For vampires, I think its "drain Life spirit" that works that way too. Thats what I try with papparazzi anyway.