This is a habit we have to break nearly every Tailback of every season and it is fairly simple to do. Most teams are very successful using the following approach:
Start by explaining the off-tackle plays and showing where the BB is going to block. Demo where the defensive end will be blocked and show that it will be right into the ballcarriers path if hthe TB chooses to run outside. Chalk it and walk it and specifically talk about running the ball inside.
Preach that you will not move past the 16 power until it is run perfectly 19 out of 20 times. Talk about the fact that if a player decides on his own to run it to the outside, he will be taken out of the game and not be given the chance to run the ball again for a very long time. Explain that you have a play called the 18 sweep that is used for the specific purpose of running outside and if you see that the 18 Sweep play is open you will call it. Stress that 16 power will work and you are going to run it no matter what, to set up your other plays. Stress that even if it doesnt work, it is setting up another play in your arsenal.
When you rep the plays on air with dummies at the Point Of Attack, station a coach with a shield just past the coach playing DE with a shield. Let the players know that on the 16 power if anyone tries to run the ball outside that DE that the coach with the bag is going to pop him. Rarely do you have to do it. Flat out do not give your players the freedom to read that play. If the DE is squeezing down, you will call an 18 sweep.
If a player goes away from this hard and fast rule, sit him down. We see many first year teams that have this problem. In 2004 my select team had a real stud TB, he was very good, in the very first game he ran a power for a 30 yard gain. the sidelines and the stands went nuts. Internally I went nuts too because he ran outside the kickout block. I got him to the sidelines immediately, he knew what he did was wrong. I gave him the look and he sat the bench for the remainder of the quarter. Needless to say it never happened again and he ended up having a sensational season.