1994 Chevy Prizm plugs keep fouling out

Engine Mechanical problem
1994 Chevy Prizm 4 cyl Two Wheel Drive Automatic

looking to buy a 1994 prizm, man bought car from some one that needed money. He took it to his mechanic, he did tune-upbut the plugs keep fouling out 3rd set of plugs. What could be causing the plugs to keep fouling.