How about how to stop advanced rippers like Amuri for ripping your theme?
It's pretty much impossible to stop a decent ripper; you can make life difficult for them, but if someone's dedicated enough, they will be able to rip your theme.
My suggestion would be to continually work on your theme, so that any ripped copy is always out of date.
The more you change your theme from default MyBB, the less likely it'll successfully be ripped.