It usually works every time for me - if a castle is under siege by your forces you can withdraw from the province and so long as the besieged enemy doesn't sally, contact is broken. Obviously it depends on the besieged faction's entire army being in that castle, and the presence of any ships can also maintain contact if they are in a sea region next to one of your territories or a sea region you control.
I've done this many times with the Pope to keep him holed up in the Papal States.
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