Can I turn off one remote if I no longer want that person to have entry?

It can be done but the convenience of it depends on the remotes that you have. The remotes that come with all of our gate openers are designed for residential usage where you have a small number of remotes given out. In this case there are dip switches inside all the remotes and on the receiver that must match. If you change a single remote for the receiver that remote will stop working. If you cannot get that remotes back you should change the dip switches on all the other remotes and on the receiver. 

We also carry Linear Mega-Code remotes. These remotes are used in communities and in businesses. Each remote has a remote id. The remote id is then programmed into the receiver. The receiver can then cancel a single remote id upon command without having the remote present.