Open/Unused player(s) question

In a map I created a builder for each of the 8 players. But sometimes not all 8 slots can be filled. Is there a way to remove the extra builder for a player that is unused/open at the start of the game ?

Besides, I've another question :(.... For dialog triggers, how do I detect which player clicked the button ?

Thanks.

For the first one use a player slot status comparison.

If (player X) slot status equal to is playing
then (give him a builder)
else (don't give him one)

OR
Event: Time elapsed is 1 seconds.
Condition: none.
Actions:
Player group - pick every player in all players and do
If (picked player) slot status equal to (not playing or has left the game)
then - (remove his worker)
else - do nothing



I don't know anything about dialogs, sorry :(