Remember that there are generally two accounts involved in staking - the stash and the controller account. (Learn more here.)

In order to stop nominating, you need to have your controller account loaded in Polkadot-JS to perform controller-based actions such as stopping nominating. If you're unsure which one is your controller account for your stash, you can see all of your stash/controller combinations on the Account Actions page. Your controller will be listed right next to the stash account. 

You can click on the controller to see the address in the panel that will open on the right. Or you can click on the DOT icon in front of the controller to copy its address directly. 

Once you have found the relevant controller, make sure it is also loaded into your accounts page. If you can't see it there, you can restore it from your mnemonic seed or JSON file or attach your Ledger, if that is where you keep this account.