It is very difficult to deal with someone spreading lies and misleading information about you. Keeping in perspective is easier though if you keep in mind that most people will know what your mother is like, at least anyone who knows her and is not just a casual acquaintance.
The stress of coping with someone with a personality disorder is high. To help yourself, do some relaxation exercises. Offset the stress with treating yourself with special outings or vacations. Make yourself feel special when you can and ask for help in doing so from your partner or friends.
Also, if anyone mentions your mother or alludes to something she said about you, you can always downplay it by saying mom has some problems or something similar. Don't try to defend yourself or go into long explanations of her problems. The less importance you put on it, the more others will dismiss it.
You can also tell her to stop, but as you know that could go either way with her response. She may use it against you or she may be frightened off. There is no way to tell for sure.
Trying to relax, talking to others and relying on family to help you can also relieve some of the stress. Ask for support from those who know your mother to quell some of the rumors. It is better the denials come from them rather than from you.
You can work on making your mother's visits shorter, one step at a time. Next time you see her, cut off an hour. Slowly backing off is just as good, and maybe better, than cutting her off all together.
And keep in mind, most people take rumors or gossip from others like your mother in stride. If they are good people and caring in nature, they will dismiss her talk until they have proof that it is true. If they do not, they are not worth your time anyway.
This resource may help you communicate with your mother so you feel more in control: