There's a freezer spell posted on this site:
I think it's a very weak version of a freezer spell, but it's a start. I would give the term a google and see what you come up with. Spells like this which use personal effects such as hair, handwritting samples, or a doll-baby effigy/representation of the target, etc., are the most effective. You can also incorporate herbs and other botanical helpers, etc., to strengthen the work. Some people freeze their enemies in a container of urine for a little extra "interest."
I think a freezer spell would be a good starting point to keep this ex-girlfriend "on ice." This is one example of a family of spells called binding spells, magic which works to control and limit another person. It might not be enough, though, but it's a good least-restrictive first attempt.
Another angle would be to work a blessing spell for her to find her soulmate - not your current boyfriend of course - and fall madly in love with this new person, thus leaving the two of you alone to enjoy your relationship without her influence. That would be the kinder, gentler approach.
As always, all magical situations have two sides to choose from.