The on MODE event triggers when a user changes a channel mode.
Note that it does only trigger when a channel mode is changed, not when a user mode is changed.
Remember, that you need to specify a limit to set +l and a key to set and unset +k, but you dont need to specify a limit to unset +l. For additional info, read raw 005.
To reverse any changes a user makes to the channel, take a look at this script.