When turning your shower's faucet handle no longer cuts off the flow of water coming out of the showerhead, you need to disassemble the faucet and inspect the handle as well as the valve stem or cartridge. Stripped splines inside the Moen faucet cannot control the flow of water until you replace the damaged parts.
Your Moen shower's valve stem or valve cartridge fit inside the faucet's water volume control handle, which you turn to increase or decrease the flow of water. Both the inside of the handle and the portion of the valve stem or cartridge that fits inside the handle have a series of interlocking splines, or fine teeth. When you turn the faucet's handle, the splines turn the valve stem or cartridge, changing the flow of water inside the faucet.
The splines on the faucet's valve stem or cartridge, as well as inside the faucet's handle, don't last forever. Since you change out the cartridges completely when the faucet starts to leak, the cartridge splines may not be as likely to strip. With valve stems, you may keep the stem and only change out worn parts when the faucet leaks, meaning the valve stem has the same splines as before. As you use the handle, the splines wear down until they finally no longer interlock. When this happens, turning the handle no longer controls the flow of water as it did before.
If you pay attention to how your Moen shower faucet operates, you can catch a problem with the splines before it reaches the point where you cannot turn off the flow of water. Warnings signs that the splines in your faucet are wearing out include the handle turning loosely or wobbling on the faucet, even though the screw that holds the handle to the faucet is tightened all the way. Difficulty turning off the faucet all the way indicates a problem with the splines.
You must cut off the flow of water to the faucet once the splines strip out. Close the house's main water valve, which is located in the basement, near the water heater or at the street. Back out the screw holding the faucet's handle in place, then remove the handle. Looking at the inside of the handle as well as the end of the valve stem or cartridge allows you to see which splines are damaged. Take the damaged parts with you to a plumbing supply store to purchase exact matches, then install the new parts in your Moen shower.