If you having this issue and have already gone through the whole bypassing of driver signing process AND you have installed Makerware at some stage try the following:
Load Makerware, Select Services, Stop Background Service.Close Makerware
Then Load ReplicatorG and Connect to your printer.
I had same issue with a Mbot Cube 2 (old original motherboard 7.2 firmware) and Mbot Cube Single (7.5 Firmware), but NOT when connecting to a Prusa I3. Unsure if any other Mbots will have same issue.
Have not found how to kill it permanently. Will provide feedback when I do or maybe someone else will.
Solution found in this post #5. (http://forum.arduino.cc/index.php?topic=269215.msg1898431#msg1898431).