I don't think filtering the spur is a good answer; it's sort of brute force.
Better to find the cause and eliminate it.

If the oscillator is on 40 meters it would be strange to find a subharmonic.
On the other hand, if it's on 80 and there is a doubler, that could explain
it. (I haven't looked at the diagram.)

Assuming the oscillator is on 40, the 80 meter spur might occur because the
oscillator is squegging, or cutting out, on alternate cycles. Or, if it's a
crystal, the crystal might be fractured.

If the oscillator is on 80, reduce its amplitude and investigate the Q of
the succeeding circuits.

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

73, Bob K6DDX