How do propeller-driven, fixed-wing UAVs counter negative torque from engine rotation?
In exactly the same way as any conventional, single-prop aircraft does

Which is to say, by using stabilizer and aileron/rudder trim, and the natural resistance that wings/stabilizers give.

The wings/stabilizers will naturally resist rolling forces to some extent, particularly at flight speed. At lower speeds (eg when taking off or landing) the control surfaces can counter the force quite easily - a little opposite rudder and aileron is all that's required.