Remove files from a folder not used in Premiere Pro
Do you want to delete footages, which you didn't use in Premiere or you even didn't import them in Premiere?

In first case I don't know how to delete them.

In second case I think that you can look at the names of cache files, which premiere have created, then open program like “Beyond Compare”, compare “video folder” with “cache folder” by names. And then you should get, which names don't have in cache folder.

This names will be not used. If your cache are full, you will have full list.

Maybe this idea will be useful for you, but I am not sure.