This is a duplicate of a thread started as
[Fixed 9sp1b2] SP1 - Still can't clean up Open With menus on 30 Oct 2013. There are fixes described in that thread. One worked for me: I had to open the tpdb8 (MS Access) and find the records in the UserSettings file that dealt with external programs. When I deleted the 11 records and reopened TP, the spurious external program entry was gone. If you have more than one external program set up, be careful: the external program records in the db table are keyed to *numbered* external programs, and I don't know what might happen if there is an external program 1 without an external program 0.
If you don't have MS Access, there may be some free viewers available online. Not sure if they will also allow record deletes.
TP developer Phillip posted that the bug had been fixed in TP9SP1beta1, but I have the current release installed, and his description doesn't match my experience.