I have an interface (just a simple interface, not part of a bridge) and I would like to throw away incoming packets from a specific MAC address, can I do that on a RB960PGS (hEX PoE)?

Most likely switch ACL rules.

Check this thread for more background info.
viewtopic.php?t=203589

On that particular device model, the following should work but you have to try:
/interface ethernet switch rule
add ports=ether5 src-mac-address=10:20:30:40:50:60/FF:FF:FF:FF:FF:FF switch=switch1 new-dst-ports=""
On devices without switch chip rules support, you would have to create a dedicated bridge whose only port would be the interface in question.