The jet fighters could harass the small plane, to the point of making it very clear: You will not get much farther on this path!
Note what happens when the F-14s fly under the Zeroes here:
The Zeroes aren't waggling their wings. This is an upset - they are being flipped over by the wake vortex from the Tomcats. The competent pilots are then using aileron to arrest the upset.
The same effect is examined in great detail here. In both cases the Zero and DR400 are able to arrest the upset at 90 degrees, but tragically the DR400 is much too low. Its wake turbulence was from an An-2 biplane (1/4 the weight of an F-16 and 1/13 the top speed) which had taken off 40 seconds earlier. TURN YOUR SOUND DOWN:
Anyway, 2 or 3 fighter jets could hit this guy with that kind of upset every few seconds, and from different angles, diving for instance to put him in a spin. He'd be at 100% workload simply recovering from upsets - he'd be so busy trying to put the horizon line back where it belongs, that he'd have no chance of even knowing which compass heading he was flying! As soon as he's pointed away, the fighters leave him alone, and if he turns back to his heading, suddenly green and blue are sideways again!
Inducing these upsets is inherently rather dangerous, so the fighter pilots wouldn't do it unless deadly force was authorized.
Further, the fighter pilots would have a real job matching their harassment to his ability to recover. They could very easily create an upset which was totally recoverable, just not by that pilot. The longer they have to do this, the better; ideally they'd start by throwing "softballs" at him, and if he handles those easily, turn up the heat until they find his skill limits.
What if the fighter is unarmed?
Obviously, the fighter jocks could induce these upsets with an Antonov-2. So it's no trouble in an actual fighter. They have even more options, even without weapons.
The fighter could make a high subsonic or supersonic pass from behind very close, inducing such extreme turbulence as to snap the wings off.
The fighter could intentionally collide with the light plane's control surfaces; the 30,000 pound fighter's wing leading edges are far less likely to take critical damage than the 2000 pound small plane's trailing edges.
Finally, the fighter has the "blank" ammo they carry for weight and balance reasons.
Fighter jets don't shoot bullets, they shoot shells. Shells have a fuze and bursting charge, which explodes the shell into a cloud of shrapnel, doing devastating damage. This is necessary due to the speed of modern aircraft. Fighter jet "blank" ammo has propellant, but no fuze or bursting charge, it's just a slug -- so yeah, it's a bullet. It passes through the airplane's structure, leaving 2 or 4 holes and wrecking anything in its path. Usually nothing, but if it gets lucky, it hits a control wire, spar just right, human or the engine. At 50 rounds per second against a 150 mph foe, they don't need to get all that lucky.