Stopping it from growing won't be any problem. Most people call it stunted growth. The only problem is that the insides don't stop growing and the fish will die.
And a lfs will tell you stories about how 4 RBPs can fit in a 29 for 2 years without any problems (speaking from experience) even though they can reach 8" within the first year. I moved mine into a 75 after having them for about 3 months at which time they were about 4" and I'd say that they have grown about another inch in the past month (which, I feel, wouldn't have happened if they were still in the 29).