Are there cases of a thrust reverser used on a turbojet engine?
Yes.

The Rolls-Royce/Snecma Olympus 593 [Wikipedia] were pure turbojets and they had thrust reversers.


Source: Wikimedia

Also, T/R on Concorde were also used in flight (If I remember correctly, only the inboard ones were used in flight) because Concorde didn't have spoilers.

Being a civilian aircraft, with the engines mounted under the wing, Concorde didn't have any real problem with thrust reversers