Separator Block

Can be crafted with an observer in the middle, one redstone dust on each side of the observer, and a hopper below the observer.

Basically, you place the block against any storage container, such as a chest. Then, you interact with the separator and it opens like a hopper. Inside it, you place the items you want to go into the chest. From that moment on, no other items besides those placed inside the separator can enter the chest or storage container.