what caused george to stop playing mean-spirited jokes on lennie?

George tells Slim that he realised that it was not appropriate to tease Lennie any more when he almost loses his friend.

George told Lennie to jump in a river. Lennie followed George's instruction because he trusted him. George was betraying that trust, and Lennie nearly drowned as a result-

'He damn near drowned before we could get him. An' he was so damn nice to me for pullin' him out. Clean forgot I told him to jump in.'

George realised that he was abusing their friendship and was not showing the loyalty to Lennie that Lennie showed him-

'Well, I ain't done nothing like that no more.'