Punishment is a corrective measure that hopefully will only be needed on rare occasions.
For first offenses, the punishment may be mild and considered to be more of a teaching moment.
Repeat offenses need to be dealt with more harshly. At that point he is considered trained, so in addition to the original misbehavior, there is also a punishment for willful disobedience.
Before any punishment, be sure to tell him “I am disappointed in you.” The worst punishment by far is the shame he feels inside for displeasing you.
Punishments shouldn't be fun.
It should be something he genuinely doesn't enjoy. It’s a punishment, after all, not funishment.
Correcting your man’s bad behavior doesn’t have to involve pain.
For example, you could…
Make him stand in a corner.
Make him exercise (if he’s healthy enough to do so).
Make him do extra chores.
Make him handwrite repetitious sentences 1,000 times.
Remove privileges or take away something he likes.
Ignore him.
Withhold sex.
These are just a few of the tools you have at your disposal. They are all effective.
And if he continually disobeys you, you can threaten the nuclear option of ending the FLR. After all, he’s supposed to be serving you, not adding stress to your life. If he won’t obey what’s the point?
Also remember to reward him when he’s good. Men in FLRs like positive reinforcement when their service pleases you.