Disable the mouse on the tablet but allow the use of the pen
Actual Multiple Monitors (http://www.actualtools.com/) (as recommended by DisplayLink who provide the video driver used by many of our business-oriented tablets) includes this feature.
Their Web site has a description of the feature. An evaluation version of the tool can be downloaded free of charge but a fee is payable for a permanent licence.
Please note that we cannot provide support for Actual Multiple Monitors or accept any liability for its use - please contact the company in question directly for any queries or support needs.