support answer:
First, make sure you have the Advanced settings turned on (by pressing Ctrl+Alt+D). Click on Advanced, then click on Show Debug Settings. That will bring up the Debug Settings window. Either using the dropdown box and scrolling, or typing into the bar next to the dropdown arrow, go to the setting PitchFromMousePosition and set that value to 0 (zero). Next, go to the setting YawFromMousePosition and set that value to 0 (zero). It can take a few moments for the setting to take effect, but once it does, your avatar should stop following the mouse cursor as long as both of those settings are at zero.