Moto Gahreis has a few accumulated rulings... Go
figure... Block Supply lines is always legal
to play (if he's attacking, of course), as the effect,
by itself, WILL remove him from the battle. If Gahreis's
controller then stops that (Test of courage, or right
hand, for example), that's a seperate effect.
Destroy cards, even if UNCONDITIONAL, are not legal to play.
Even if you wanted to play a
Peasant Vengeance,
it would not be legal, as the game makes a distinction
between "Destroy" and "Remove from the battle".