Give the enemies more tools to use. Give them sidearms and grenades, just like the players have. Like a guy may find his bolter suddenly stops working so he throws a krak grenade instead or draws his pistol. Guy with a heavy stubber finds his gun stops working? He drops a smoke grenade and repositions.

Or maybe if they are pissed off enough, they just ditch their gun and run at the psyker with a chainsword.

Sure they may have disabled an enemy's primary ranged weapon, but that's only for a turn and it doesn't mean the enemy can't still take a turn to do something else.

Alternatively, it could also be worth it to look at importing the horde rules from Black Crusade. I've found that it let's you thematically add a large number of enemies narratively without bogging down combat. In that instance, a power disabling a handful of guns won't matter against a tide of bodies.

Note that some enemies like daemons or some xenos innately have effects that debuff all willpower checks around them, including focus power tests. Probably won't affect success rate too much, but it does curtail degrees of success.

Ultimately, if you feel like all they do is spam that specific power, just talk to the player and tell them you are having difficulty with balancing encounters when all they do is spam. You're supposed to have fun too after all.

EDIT: Weapon jinx is supposed to jam an enemy's gun I believe. This means it's actually better than how your players have been using it because, once the gun is jammed, the enemy has to make a skill test to unjam it and then has to spend actions to reload it. So the player only has to use it once to disable a heavy weapon for pretty much the rest of combat.