My ex tried unilaterally changing the court order. This was several years ago. Our order had the summer split 50/50. Ex sent a proposed summer schedule that was closer to 70/30. I proposed week on/ week off. She said no. After three proposals by her I filed a petition to the courts to have the summer schedule resolved. The day before the hearing ex set up a mediation to try to work things out ? I sat there and was told how abusive I am , etc. Finally I sttod up and said I will see you tomorrow. Went to court the next day and she had a proposal that met the court order. Issue resolved.
She made several minor issues after that and my email pointed out the court order. Sometimes I would get a rambling email about how abusive I am or something similar. However, I stood firm with the court order and we were able to work out an agreement.

I agree if it happens pretty often I would go to court. All you are doing is establishing a boundary but you need the courts to back you up. You may be able to do this pro se. You will need to get some advice or guidance.
If it is not very often you may want to send an email each time (documentation). After a few you can then file a contempt of court. Having more than one incident will help in court especially if it happens in a short period of time. Once a year will not be worth it but several times every four to six weeks will get the courts to help.
Several years ago I had one incident. Our S11 at that time had a birthday. I went to pick them up to take them out. The court order gave me three hours on their birthday. When I went to pick them up I was told that S6 didn't want to go. I wasn't expecting this and couldn't think of what to do so I left with just S11. When I returned S6 came running out and wanted to know what we did. Of course, I suspected ex was pressuring him beforehand. I told him what we did and I could see he was disappointed. He never let that happen again. I came to expect these kinds of things after a while. What I didn't realize at the time was how these incidents eventually backfired on ex. Both boys have learned to not get pressured by their mom anymore about spending time with me. Along with all her other behaviors she is pushing them away more and more.
Figure out ways to work around the roadblocks. Sometimes court is the only way.