Remove plugin

No, it cannot to be turned off via ftp. If you are having trouble with login, you can temporary rename the folder. if you want to turn off you can login from another admin account then select edit user that you want to turn off, there is an option to turn off on user edit screen.
If you are site administrator and lost access to your wordpress account, you can turn off plugin by modify database on user_meta table find the field wp2sv_enabled and clear it’s value