I may not be much help here because I do not show halter horses, but I do lead my horses and will not tolerate them walking on me. So that said here is what comes to mind for me -
First thing, make sure you give the horse plenty of lead. Many beginners seem to choke up on the lead and hold it right under the horse's chin. You want a minimum of 8 inches of line, aim for slack in it while you walk. I like to see how much slack I can get while training them, aiming for them to lead almost 'at liberty' with the lead thrown over my shoulder and no pressure on it at all. Second thing is I don't want a horse walking so close they will step on my feet or knock me over with a small startle so with those who want to do so I start carrying a short crop (a ruler would work as well) in the hand nearest the horse. If the horse crowds my space I will bump them in the neck with it to move them over. If they really don't understand the cue I will practice standing still asking them to step away from the pressure of the crop. Start with steady light pressure working up to as hard as needed to move them. Remember that horses instinctively move INTO pressure so you may have to hold pressure and wait a bit until they step away. Reward even a relaxing away from the pressure at first, with a release. and then walk a step or 2 and do it again until the horse understands that when you touch them with the crop they should step away from it. This will help when you begin training for halter obstacle or driving too as they will know to step away from pressure even light pressure. Once I am sure the horse understands they are not to walk too close I will get quite firm with them if they crowd me, I consider it a safety concern and as you noted a matter of respect. Time should cure this problem if you are consistent in your requirement that they give you space.