That brass cap will stop that little low pressure drip you have on the valve's Male threads. But in the real world under pressure, it would need a washer/ gasket placed inside of it before installing it. You're just pressing the brass against the rim of the shoulder where the ferrule would sit. You can always take the handle off the valve too to help from someone accidentally turning it on.
Get a compression nut and ferrule and cut a short piece of 1/4 inch round brass rod. Install the rod like it were tubing. Also you could create a blank from copper tubing, smash the end flat and solder it closed.
Teflon tape on the threads does nothing here, the seal is made either metal on metal with the ferrule or for pre-made flexible lines like installed there is a rubber seal in the top of the nut.