If you can't uninstall it using package manager then probably you still have some nodes in the flow, or configuration nodes. Check in the menu Configuration Nodes for any unused nodes and delete them and deploy.
Alternatively to uninstall from the command line go to your .node-red folder (that is important) and run npm remove node-red-contrib-ui-i
If that doesn't remove it then post the full output from the command here.
Manually delete what folder? If you mean the you deleted the folder from the node_modules folder, but have left it in package.json, then npm is liable to re-install it for you without warning.