It sounds like the throttle cable needs re-adjusted or the microswitches in the black box are not clicking right. The 2 micros in the black box do the following..
1st micro clicks, and removes the ground from the coil. If this is not returning to its proper position, it will not kill the coil. So, any fuel still getting by the butterfly will cause it to run when you let off the gas. And will also cause a back fire occasionally.
2nd micro. When clicked, sends 12vdc to the selenoid and makes the starter turn over. If this does not return to the right position when you let off the gas, it will cause the starter to stay spinning and give you the impression that the cart is still running.
To check the micros, remove the cover of the black box and with key off, slowly press on the gas and verify 2 clicks. When you let off the gas, you should hear 2 clicks again. If you are not, then loosen the 10mm nut on the throttle cable going into the black box and put a few threads into the black box and check again.
If your happy with slow speed and acceleration, then you could just bend the tabs on the micro a little to fine tune them the way you like.
You should be able to press on the gas and literly start the cart without moving. There is a difference between starting and running so listen to it carefully. Then, anymore acceleration and then the cart moves. This is the ideal setting. And if you have head lights, make sure the starter micro clicks at the same time as the ground removal micro. Or you may run into charging problems at night. Lights dim at idle then bright when going 5mph can cause battery drain. My camp ground speed limit is 5mph. I had miss adjusted the micros once and the battery would go dead only at night. Found out it was my micro switches adjustment.
Hope this helps.