I think there are a lot of factors to consider in this type of answer.
1) How long have you been friends with the ex?
2) How long have you been with your current partner?
3) What is the issue? Jealousy? A question of time commitment? Still have feelings for the ex?
If it was a question of jealousy, then I would want to figure out if it was justified. Is my partner insecure? Am I causing that insecurity? Or is it just a matter of trying to control who I hang out with? I don't think any situation is clear cut. Sometimes, it might be a good idea to break off that friendship with the ex.
However, if your partner is trying to control you and there's no good reason to justify the jealousy, I would be inclined to question the current relationship, rather than cut off a potentially solid, platonic friendship.