No matter if it is on the phone's "Internal" storage, or the SD card (which is "External" storage), they can easily be removed. That is if they are standard files (music, movies, etc.). Now if you are talking apps, that requires they be uninstalled. If you are talking something else, virus or malware perhaps, then that would require the right app/software to remove them.
To remove files from the storage ("Internal" or "External") you just need to locate the file. Most files can be found through the apps that use them. Gallery for images, etc. or you can try using the devices "File Manager" app. Most devices come with such an app. If yours does not, then you can install one as well. This will allow you to locate and remove files from...