Remove a part of a champion's passive
I like a lot of champions so i'll list off a few
Nimble Fighter
Fizz perpetually ignores unit collision and takes 4 / 6 / 8 / 10 / 12 / 14 reduced physical damage from basic attacks.

Chosen of the Storm
Volibear periodically regenerates 30% maximum health over 6 seconds when below 30% maximum health.
Adrenaline Rush
Dr. Mundo regenerates 0.3% of his maximum health each second.
Talon's basic attacks deal 10% increased damage to enemies that are slowed, stunned, rooted or suppressed.
King's Tribute
Whenever an enemy dies near Trundle, he heals for 2 / 3 / 4 / 5 / 6% of their maximum health.
Blood Well
Upon taking fatal damage, Aatrox enters a stasis and drains his Blood Well, healing himself over the next 3 seconds for up to 35% of the Blood Well's maximum capacity, plus its stored health upon activation.
After casting
Poppy halves non-turret damage against her~~ that exceeds 10% of her current health.~~
Upon taking fatal damage, Karthus remains active 
Damaging spells afflict enemies with Sunlight
Braum adds stacks of Concussive Blows to enemies
Soraka runs faster ~~towards nearby low health allies. ~~ (Hecarim and Rammus, you have competition)
Lissandra's next ability