There are a bunch of ways to do it, such as setting the melee turtle to constantly detect a redstone signal (I suggest that you make it only attack when the redstone signal is ON, so if the lever is broken it will default to off. For bonus points, tie it into your T5 spawner's redstone controls), make an access port away from the face of the melee turtle, or use a remote computer (using the rednet modem API) to start/stop the turtle's attack phase.
If you mean that it is stuck in the program, hold down CTRL and then hold down the T button for two to five seconds to break out of the program.