Sexting with an ex😳

So surely I'm not the only one whose done it, buuuut.... When is enough, enough? My ex and I have been sexting and as fun as it is, I want it to stop. Every time I try to put a stop to things we some how end up back at square one! We don't wish to get back together so I don't understand why we can't stop? Hellllllp!! I need ways to completely close this door as it's going to bring me nothing but bad karma!