You could've stopped them by breaking off the plastic covering the read-only hole, it would have prevented writing to the tape. And, then when you were using the tape and wanted to write to it you could cover it with adhesive to make it writable again. As long as they weren't aware they could do the same, your tapes would have been safe. Because, the moment they went to record over it they'd think the tapes were defective or not compatible.