I had an Axman get DFA by a Nova, the displacement should push it off the edge of the map depending which rule book you read, Total Warfare or Mech Manual.
According to the Mech Manual (newest printing) p.37 DFA Prohibited Terrain basically says a 'Mech displaced off the playing area destroys the target.
Total Warfare (newest printing) p.150 DFA Location After Attack says "If the target unit cannot be displaced into the appropriate hex (for example, if the terrain is prohibited), the hex chosen must be as close as possible to the original hexside through which the target unit would have moved. Page 151, Unit Displacement also does not mention a unit being able to move off the map.
Attached is a picture from the game, red arrow is following Mech Manual, green arrows following Total Warfare.