Some flash drives have tiny switches on them to toggle Write Protect on and off but if yours doesn't have one, use your Windows key and R together to bring up the Open box and into that, type regedit then press Enter.
From the File menu, choose Export then note the name and folder of the backup you're about to create - it could be crucial if anything goes wrong with the Registry.
In the same way as you would in My Computer, navigate to this path:- HKEY_LOCAL_MACHINE\SYSTEM>CurrentControlSet>Control>StorageDevicePolicies and change the Value from 1 (one) to 0 (zero). Exit through the File>Exit menu and restart the computer and you should then find that the stick will behave properly.
If the key StorageDevicePolicies is not in there, you will need to create it. Post back if you need to know how.