I have just finished my 27th BA and I can no longer do any more. I assume that once I reach Genin that I will be able to receive more gains again.
Option1
My suggestion is to make it so that you can still do Ba without getting the message that everything is capped.
By adding a extra attack to the Ba, For example, Fake Punch, we can still gain the 30 exp at the end and the AI battle goes up. No gains would be received except for ryo. No stat gains.
Option 2
Set the gains at a certain point level. depending on the level of course. This would make it more balanced. Same type of stat gains but Garenteed amount of gains.
Just a suggestion of course. feel free to add your input.