Horse body-slams me, how to train?

When you go to untie him and he slams you he's saying "F you!" basically. So you NEED to show him whose in charge. My horse did that to me once and he learned it wasn't a good idea. Depending on what halter you have. I took this one horse out and I used that owner's halter and he slammed into me and i yanked the lead line and it did nothing. So I got my halter and he tried it again and I yanked it and he learned quickly. If you don't have the lead line in hand. Bump your should into him to back him up. Depending upon which side you are on. It's hard to explain over the internet. Your horse thinks it's funny. What i do is if he wants to act disrespectful just leave him tied there all night until he stops his attitude. He will think twice next time. If he still doesn't leave him there for however long it takes. Some people will disagree with me on this, but that's ok.