One way to go about it would be to give the judge two options to make an arrest. Option 1 would be something like "Ask noble to surrender," which would give a noble an amount of time to surrender, and after that time had elapsed militia would attempt to arrest the noble in question. For simplicity's sake, the noble would be able to surrender in any region of the realm.
Option 2 could be "Attempt to arrest noble" which would use the Judge's unit to immediately attempt to arrest a noble. Chances for success would depend on the size and strength of the arresting unit vs the resisting unit. Add a chance, depending on the status(militia should never aid in the arrest of a Royal, and militia should be more likely to assist in the arrest of a unaligned knight than a Duke) of the arrestee, for the militia to join in on the arrest. If a Judge attempts to arrest a Lord within that Lord's own region, then the militia of that region would naturally aid their Lord.