llUnSit(key id)

If the agent identified by id is sitting on the scripted object or is over land owned by the scripted object’s owner, the agent is forced to stand up.

Notes:

When dealing with group land, the object owner must be the group. It won’t work if the object owner is a member or officer in the group; the object itself must be deeded.
Another way to prevent sitting completely is to deny access to land. People can still visit any building above the height limit for which land access applies (the red stripes), but will not be able to sit at all.
The tool tip for this function indicates that key is returned by this function, but the compiler won’t accept this.
After llUnSit(av) a granted PERMISSION_TRIGGER_ANIMATION is lost.

Compare with llEjectFromLand and llTeleportAgentHome.

Q: When I make an avatar stand up from my chair, they end up standing on it. That looks stupid. Is there some way to get them to stand up and move to a specific position?
A: Not precisely. There are a couple things you can try. The most effective option is to apply a small force to the avatar with llPushObject. Alternatively, try having the chair turn phantom when they stand up, so as to make them drop to the floor. They still might appear to be standing “in” the chair however.
A2: Try making an invisible object in front of the chair as the master prim of the chair that has the phantom property and a script setting the offset position back to the chair (using llSitTarget). When the avatar will stand up, it will appear at the object’s position in the linked set whereever he clicked on the linked chair.

Q: How do I force an avatar to sit on an object?
A: You can’t. You can only force them to stand up. It is possible to animate an avatar so they look like they’re sitting, but not to control their position with much precision or reliability, at least not without using attachments.