Here's how you nerf Tahm properly

Now, I'm someone that's been playing Tahm ever since he was released. I play him top, sup, and sometimes jung. I don't want to go into a long drawn out analysis of Tahm, I just want to get straight to the point.

The way you nerf Tahm is by hitting his mana costs, and mana regen. While the game my suggest you buy Roa for your first item on Tahm, it is really unnecessary as your mana mostly go away once you've hit level 6. Make it so itemizing for mana is more important for him by making his Q increase in cost with ranks and bringing down his mana regen (Yes I know he already has one of the lowest in the game).

This will make it so spamming abilities is harder due to risk of going oom, it will make it so itemization can be harder since mana and mana regen items aren't optimal on him, Frozen Heart gives no health, Roa gives no resistances and the mana regen items don't provide tanky stats for him. If he does itemize for mana then he can more freely use his abilities like he does now but he'll still be giving up something for that.