Agree with gym miss - she cant stop you! Get some legal advice and force her to. She's really using them - I assume they want to see you - to get at you. This isn't good for the kids either, it's bullying. If she has a problem with maintainence then she can have legal redress to that too.
Have you paid the maintainence you owe? If you cant then maybe that is something else that you need to look at.
I'm divorced and my ex uses all sorts of excuses and reasons to trying to manipulate me into having the kids when she wants.