llForceMouselook(integer mouselook)

After calling this function with mouselook set to TRUE, any agent sitting down on the prim will be forced into mouselook.
Just like llSitTarget, this changes a permanent property of the prim (not the object) and needs to be reset by calling this function with mouselook set to FALSE in order to disable it.

Note: This only affects agents who sit on the object after the function has been called. Any current sitters remain unaffected until they resit.

The agent is forced into mouselook upon sitting on such a prim, but can exit mouselook on their own afterwards. It’s useful for vehicles and the like.

Q: Great, but how do I force someone out of mouselook?
A: Unfortunately, you can’t.