Instantly silences target for 2.5 seconds and instantly burns x mana. OR
Squelch: Reduces a Champions AP by 20 + x/lvl for 4 seconds and burns 50 + x/lvl Mana AP(x) I think should be something like 10 and Mana(x) should be something like 15. (Kudos to Gale for the idea)
Mastery boost = next spell casted after being affected by squelch has 20% increased cost for 4 seconds.
Edit: Short ranged spell (300 range), to prevent from casting it just to burn mana.
Because melee shouldn't be the only ones shut down by a summoner.