Are there cases of a thrust reverser used on a turbojet engine? In the case of a turbojet (with no "cold" bypass flow), the airflow being directed forward by an hypothetical thrust reverser can be really hot.

Depending on the location of the engine, this flow may be dangerously redirected towards the empennage and the fuselage.

For instance, for a fighter this would be a hazard for sensors, equipment, payloads including missiles, likely many other elements, and pilots:


General Dynamics F-16C (source: Wikipedia)

Are there turbojet engines fitted with a thrust reverser? Which aircraft are they used for? How is this problem dealt with?
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