Actually, it depended on the scenerio...
If it was a fixed map, then you could force someone off the side of the map, and it is clearly stated in the scenerio description "any unit that leaves the map is considered to have disengaged and blah blah blah. Sometimes, there are limits as to the disengagement direction, and if you tractored them off in one of those "wrong" directions, then you got a kill, not just a disenagement. Most people would therefore be leery of a tractor lock at close-to-the border ranges. It didn't help that SFB tractors rules are eviller (only the bigger guy gets impulse power and the ability to rotate, for example...)
Hence people always watched out for tractors if they were map-hugging... Nevermind that an anchor in a single internal environment was often worse than a forced disengagement...
On a floating map, then there is no end to the map, you hit the "border" and suddenly, the border shifted a uniform X hexes...
On a tournament map, there's a special barrier that would damage the poor sap that got tractored into the wall, then forcefully break the tractor.