How will Trump PUNISH the Republicans who refuse to attend his convention and/or endorse him? He won't take it lying down. What revenge will he dish out? How?

With words that cut like a knife. As President he will have assess to lots of information and I bet he will dig it up and try to ruin there career's too. Cheers and happy weekend! 

Why do you think he will punish anyone? I know that is the idea that everyone likes to say Trump os to thin skinned... Trump cannot hold his temper etc.
But his pick of Pence contradicts that narrative. Pence actually endorsed Cruz in the Indiana primary. Looks like Trump overlooked that.

A better question would be, what revenge will the GOP dish out? They are working harder than Democrats as we speak to defeat this man and they'll probably succeed.

Probably single them out and insult them