The simplest way would be to set the Keys Scaling to (0,0,0) when its picked up.
The thing is, that your general approach to this situation is wrong.
You should Have your KH player and your Key interact with each other, without the help of the level blueprint.
Instead of checking the collision in your level blueprint, do it in your KH character, if he collides, cast to your key, then you can access functions you created in your key. (OnKeyPickup|SetKeyVisibility).
ps: Don’t confuse this with the Platform Trigger Collision, the one you have there is probably working fine.