Looking at this now and I don't think that filtering will be possible without supplying a patch for the debug.ui bundle. However, it might be possible to change the name of the internall groovy stack frames to something obvious (eg- prepending with '..........').
I haven't tried this, obviously, but here's my idea just so I don't forget:
1. Register an Adapter factory for IJavaStackFrame
2. Set it to replace MonitorsAdapterFactory in the AdapterManager
3. This adapter factory will generally delegate to the MonitorsAdapterFactory, but instead of creating a JavaStackFrameContentProvider, it will create something like a GroovyJavaStackFrameContentProvider
4. GroovyJavaStackFrameContentProvider will act exactly the same as JavaStackFrameContentProvider (probably subclass), unless a Groovy stack frame is found. In that case, it will pre-pend some text to make it explicit that this is an otherwise uninteresting groovy stack frame.
I don't know enough about the AdapterManager to know if step 2 is possible and I don't know enough about JavaStackFrameContentProvider to know if step 4 is possible. But other than that, this is a bullet-proof plan.