to avoid :
to avoid avoid pax to move from club to First, best it is to be at teh gate and accompany all your first pax individually to their seat in first, therefore you know who is seating where, and also crew in the first galley can that no pax wondering around on their own to First. Most of teh time on BA, First cabin crew will meet their pax at the boarding door.
Otherwise when they try to upgrade themself from WT/P to club, big no no.... Just politly invit them to regain their seat and if they are difficult, tell them that now they wont seat anywhere as you are about to offload them!