Alright, so i have a game where it uses a timer, it works fine besides for when i complete the game, the timer still runs until it reaches 0 then my gameover kicks in, how do i make the timer stop when i have killed all 5 ghosts?

You probably have a broadcast for when the game wins.
Just add the script to the sprite with the background:

When I receive ()
Stop all scripts in sprite

I don't have a broadcast for when the game wins, how do i make it?

Select either the broadcast, or receive. Click the pull down and select NEW MESSAGE. Enter game over

broadcast [game over v]

when i receive [game over v]
stop [all scripts in sprite v]