I'd be half tempted to suggest they just get rid of jumping, except it does have valid usages like getting over obstacles or just goofing around. Horses can and do jump over fences, too, even with a rider. It's a sport.
Jumping over a guy with a big spear, though? No. Stabbing him in the face with your stubby lance while you're jumping over his spear? Double no.
A maneuver like that should take incredibly precise timing and catching the spearman off guard. If the horseman's timing was off at all, he'd still end up with a pike through his horse's leg or jammed through its belly, and he wouldn't be riding off. It sounds like in the OP, the spearman's having to line up a precisely aimed shot in a very narrowly defined area, and if the mounted guy jumps it messes up his timing and causes an impossible miss.
I think the fix would be making it so when you're trying to skewer a charging rider, timing is not a factor for you, but instead, angle. Just block, and the pointy end should be pointed wherever you're looking. If it's pointed out at a flat trajectory it'll have greater reach and hit sooner because the entire length will be extended horizontally-- raise it up a bit, and it'll hit later because it's more diagonal, but will strike an enemy that's jumping on top of you.
All in all it seems like we're lacking a 'point sharp bit toward enemy' function. There's really no way to passively receive an enemy charge, you're either swinging a weapon, actively thrusting with a point, or completely harmless. If a blocking player would just point their blade or point out toward the enemy, we'd see more conservative fighting instead of the up-close knife fights that everything seems to boil down to.
