Just make some really stupid and absurd nerf to any champion that would probably make them unplayable. Be as ridiculous as you want.

Zed:
Razor Shuriken - now only damages the first target hit.

Syndra:
Force of Will - If a dark sphere is thrown, the sphere is automatically removed.

Akali:
Shadow Dance - Now only has 1 charge, And does not replenish upon kills or assists.

Teemo- is now killed in 1 hit.
Darius-ult costs whole mana bar and has a full minute cooldown that does not refresh.
Blitzcrank- now has no q.
Cho' Gath- Feast can only be used on champions.
Garen-Judgement deals 100 true damage per second to Garen.
Lux-has no /l

Katarina:
Death Lotus: Only affects One target
:3

You said unbearable nerf. Udyr's bear form now doesn't stun targets.

Karthes's ult can only be cast when you are alive and at full health any damage will interrupt it

take that :P

Iris
"Their death is born on tiny wings"

Alright, I got this... Ahem,

IRELIA IS THE ONLY PLAYABLE CHAMP FOR NOOB PLAYERS UNTIL THEY BUY 5 CHAMPS AND SHE IS SUPER NERFED!!!

Not enough? ok

HER ULT ONLY FIRES 1 BLADE AND IT IS SUPER WEAK AND CAN ONLY BE FIRED IF YOU ARE RIGHT NEXT TO AN ENEMY CHAMP AND WILL DISAPPEAR IF NOT FIRED IN 3 SECONDS!!!

ftw...

karthus now has no fourth ability

Why not some unbearable buffs? Darius' ult is now his passive. Each time he gets a kill, the true damage increases by 500 per hit, permanently. No stacks are lost upon death. His q (passive) makes the range of his ult longer by 100, stacking by 100 per kill, permanently. His e increases in range similarly. No stacks have limits.

Nerf Rengar. Bola strike now just works to build up to Rengar's super ability. Also, give him a 10 second cooldown on his passive.

Teemo: may only place 1 shroom at any given time. Shroom no longer gives vision or slows and only does 00.1 magic damage