The correct syntax is unbind "`", but unbind ` should work. However, you'll also have to go into your config.cfg and delete the bind for `. If you don't want to play around with the config file, there's two options. The first would be bind ` to something else, like the console; bind "`" "toggleconsole". The second option is to unbind the key, leave the server, rejoin, and unbind the key again. Keybinds are stored in config.cfg and this file is executed everytime you join a server. Changing a keybind writes the new bind to config.cfg so the next time you join a server the bind is there. However, there's some glitch with unbinding keys where sometimes it takes a couple of trials before it finally overwrites the bind in config.cfg.