If it says the drive is write protected it means either it was formatted this way ..and for to see if it is so click right on it ..and choose properties and in there take a look not to be ticked on write protected ..or if it is selected read only ..remove the mark and click apply ..
If this is not a case..check the files inside not to have such a issue ..delete files one by one ..and check the ones that do not want to remove themselves..and do just like I suggested .In the last resort ..try to download and install remove on boot program ...and select tye files into the program ..and delete them on boot if they are write protected..
The action itself is to click right on the file that can not be deleted...and select to remove at next boot ..(the program will add this option on the menu ) and once selected all ..just restart the computer ..
here is the link for the program ..but lets hope it will not be needed..
After all files are removed..open the flash drive folder and under organize,,folder and search options ..click on view and select to see hidden folders ..and if there are any delete them too ..just to be sure ..
Last ..just click right on the drive's icon ..and select format ..select the options for to do that and go for it ..
here is the program I mentioned..
http://www.softpedia.com/get/System/Boot-Manager-Disk/MoveOnBoot.shtml