Identify your Apple TV remote - Apple Support
If your Apple TV remote isn't working - Apple Support
Water damage is often permanent and fatal. Short-circuited rechargeable batteries will never be the same. Corrosion will damage the circuitry.
To restart the remote or pair the remote once again, you will need to use key combos on the remote. So those buttons need to work at least, to perform the procedure.
There is no unpair procedure an sich (to have nothing paired), but you can repeat the pairing procedure. Alternatively, you could keep the malfunctioning remote out of bluetooth range, or in a Faraday cage, or wait until the battery is depleted. You could reset the Apple TV box to factory settings (optionally while keeping keeping the remote away, so that the box doesn’t try to pair with a remote).
Replacement remotes are for sale from Apple. The black Siri Remote (1st generation) is no longer available from Apple (but some third party resellers may still have a few, if you insist on that model). A second generation remote is available now to replace it, with a clickpad that support swiping gestures from a thumb. The second generation remote doesn’t have gyro or accelerometer for gaming input, so if you’re into gaming on Apple TV, you may want a separate game controller or legacy 1st generation Siri Remote.
There are no official fixes for hardware damage of the remote. (It is likely too cheap for repair technicians to work on.) Replacement is the advised remedy.