Some advice please. Should I stop my ex girlfriends contact with our children??

Im a single full time dad and have two children one non verbal with autism. Me and my childrens mother split up about 2 and a half years ago, contact has been very inconsistent since, she decided she could commit to seeing the kids once a week but still only bothers when it suits her. Almost every week I get a message with excuses as to why she cant see them or she may ask if she can come to see them, then when I reply saying yes she will ask if I will pick her up and take her back home but when I reply saying i can pick her up as im passing but I cant take her back she'll then say that she don't want me to pick her up and that she will sort transport herself but then I don't hear from her until the following week when she's messaging with some new excuse as to why she cant see them. When the ex has contact with the children she has to have contact either in my home, her mother's or take the children out to a park or somewhere as I wont allow them to go to her flat as she is in a relationship with a violent guy who cant control his anger and I wont risk my children being witnessing or getting caught in the cross fire when he looses his cool. I don't want to prevent my kids seeing their mother but I don't know how much longer I can continue to allow her to keep letting our children down, it isn't fair on them. I have threatened to stop contact if she lets them down again and ive still gone and given more chances after that which she has failed to show up for 3-4 weeks now