Which League of Legends items reduce opponents stats?

I'm going to interpret this as "items which reduce your opponents' stats". Correct me if there's a different answer you're looking for.

Reducing your opponents' armor

Black Cleaver reduces armor by a percentage on striking with physical damage.

Reducing your opponents' magic resist

Abyssal Scepter will passively reduce the magic resist of enemies near you.

Wit's End reduces the magic resist of targets hit by your basic attacks (and gives you the same amount).

Reducing your opponents' attack speed

Frozen Heart passively reduces attack speed of those around you.

Warden's Mail and Randuin's Omen reduce the attack speed of champions who hit you with basic attacks.

Reducing your opponents' movement speed

Randuin's Omen (active ability) reduces the movement speed of enemies near you.

Frozen Mallet reduces the movement speed of enemies you hit with basic attacks.

Iceborn Gauntlet reduces the movement speed of enemies in the area after you use an ability and hit something with a basic attack.

Dead Man's Plate reduces the movement speed of a character when you hit with a basic attack after charging it by moving around.

The active abilities of Bilgewater Cutlass, Blade of the Ruined King, and Hextech Gunblade will reduce the movement speed of your target. Blade of the Ruined King increases your speed by the amount that you reduce from your target.

Rylai's Crystal Scepter reduces the movement speed of anyone you hit with your damaging abilities.

Stalker's Blade (only available if you have the Smite summoner spell equipped) changes your Smite to Chilling Smite, which can be used on enemy champions to steal some of their movement speed.

Face of the Mountain grants an ally a shield which slows nearby enemies after 4 seconds.

Frost Queen's Claim creates ghosts which seek out enemy champions, revealing and slowing them.

Righteous Glory Gives a speed boost to nearby allies, then slows nearby enemies at the end of the speed boost.

Reducing overall damage

Skirmisher's Sabre (only available if you have the Smite summoner spell equipped) changes your Smite to Challenging Smite, which reduces the damage your target deals to only you and causes your basic attacks against the target to deal additional damage over time.

Reducing healing

Morellonomicon makes magic damage dealt to champions under 40% health apply Grievous Wounds, which reduces healing.

Executioner's Calling and Mortal Reminder apply Grievous Wounds upon dealing any physical damage.

I don't really see what you mean by strength so I'll go with each type of item reduction, maybe you'll find joy in one of these:

To reduce speed and atk speed:

• Frozen heart, randuin's omen, dead man's plate, rylai scepter, frozen gauntlet, hextech gunblade, frozen mallet... These one are the most common, I might have forgotten some.

To reduce enemy armor:

• Black cleaver, the two bows (forgot their name)

To reduce enemy magic resistance:

• abyssal scepter, sorcerer boots, void staff

The only things that reduce attack and ability power that I have in mind are Challenging smite and exhaust.

If you want to reduce the damage income just buy armor or magic resistance.