Apart from just deleting them from your PC- you should check the camera manual & find "format" Go thru the motions if you want to clear everything off the camera's memory card.
You should do this every time you upload pics to your PC and have them backed-up to an external disk (in that order!)
This will keep your camera card in good condition.
Most cameras have a confirmation after you press delete, something in the order of "Are you sure you want to delete these images". If you don't ok that, the camera will think that you don't want to proceed with the deletion and keep them on the card.