how to get option to disable remote control

As far as I can establish the iMac 27" 5K mid 2015 never had a built-in IR receiver and therefore as Barney-15E says there is nothing to disable. In fact I would tend to suspect none of the metal i.e. aluminium iMacs have had an IR receiver only the older plastic ones.

For those interested it is possible to plug in an external USB receiver and install a driver to add the capability to use an IR remote e.g. to control iTunes or DVD Player or similar.

No Mac currently being made now has a built-in IR receiver. The Mac mini was the last model to feature this and the 2018 update removed it.

Does your Mac have an IR port? Most newer Macs don't have one, so there would be nothing to disable.

Are you referring to Screen Sharing (the possibility to connect and operate remotely a Mac)? Uncheck Screen Sharing in System Preferences -> Sharing