I'm wondering if Ban stole the Meliodas's power forever when they fight on the arena for Diane's weapon or just during the battle.
Ban's ability "Snatch" is to steal objects without contact, he uses it to steal strength from his opponent as well. However, the strength he steals is temporary. Basically he can only use the strength until his body reaches the limit and stops to prevent further damage to himself. Therefore, the answer to your question is no, he did not take Meliodas's power forever, it was only during the battle. When the battle ends, Meliodas will have all of his power back. here is the link of reference to wikia for details. (Snatch)
Hope this helps