Some seats are decided before anything else: the head table, grandparents close to the family, the guest you promised a spot by the window. Lock them, and AI seating never moves them.
Lock one guest
Click their table, then click the lock next to their name in the table panel. Locked guests show a small lock in the guest list.
Lock a whole table
Click the table and choose Lock all.
Unlock
Click the lock again. Clear seats also unlocks everyone it removes.
When you arrange with AI, locked guests stay exactly where they are, and everyone else is seated around them.