Alright, haven't tested it, but this should work.
First go to the Character Tab in the Database and double click one of Tester's stat boxes. Mess around with the numbers in there so that rather than going up or down with level/experience, they always stay the same. Adjust the stats to whatever you like.
Second make a Common Event. With a conditional branch, do something like, "If party member "tester" is level 41, change level to 40".
I probably could've explained it in more detailed, but Im tired as hell right now, so I hope that helps.
Edit: Damn my slow typing. I suppose Trickster's method would work too. ^^