It's unprofessional, anyway this will work with javascript enabled:
document.oncontextmenu = document.body.oncontextmenu = function() {return false;}
You may also want to show a message to the user before returning false.
However I have to say that this should not be done generally because it limits users options without resolving the issue (in fact the context menu can be very easily enabled again.).
The following article better explains why this should not be done and what other actions can be taken to solve common related issues: http://articles.sitepoint.com/article/dont-disable-right-click