My usual go to for anti-summoner ops is summoning Abominations in close to the summoner. Abominations are easy to summon, and can use your opponents summoned models against them since most of them enter play wounded and relatively close to the summoner. Since Abominations strip suits from cast actions, and practically all summons require a suit, you can effectively neuter a summoner simply by moving an abomination within 2" of it. You won't necessarily prevent all summoning this way, but you will make it expensive in terms of stones and cards which will slow it down.