Make sure you have Droplet Access enabled. If you see something like below, make sure to install/enable it first.
Once done, login into the droplet, and run the following commands:
sudo systemctl disable ssh.service
to disable it on startupsudo systemctl stop ssh.service
to stop the current onesudo systemctl mask ssh.service
. This makes systemd unit files a symlink to /dev/null, so any start attempts will fail.