Your question is a bit outside the scope of MATE-desktop.
MATE-desktop can not do what you asked for !.
You will have to modify this on the displayserver-level or application-level.
Therefore you might want to ask this on the general ubuntu forum or on ask ubuntu since this question is more focused on the underlying system.
Nevertheless, get yourself familiar with xmodmap and xinput and google a bit around with these keywords to find several ways to accomplish exactly what you asked for.
Try this:
That is all I can give you at the moment.
Get more help here
And please, don't hijack other threads if you don't get immediate attention. Your message has been read already by at least 19 people which probably either don't know the answer or are already searching for an answer for you in the Xorg displayserver documentation.
Your other mail already had 89 views but no reply which means: no one knows the answer
To enhance your chances on a reply, you might explain a bit why you want to disable the right-mousebutton. A lot of people use AWS-cloud, but nobody else seems to have the need to disable the right-mousebutton. There is always a possibility that you are trying to apply the wrong solution for a well known problem. 
Again: What you literally asked for can not be done in MATE-desktop
