This sounds like nipping as resistance, My old share was a biter and I am afraid that I deal with it in an instinctive, animal way. First, I make myself look bigger by raising both arms in the air and I make a noise, a growl. The first time, I swore at the horse which got me banned from the yard for a couple of weeks. With Ella I just put my arms up and shouted No.
But one shouldnt isolate the nipping from other behaviour and compliance in general. You dont need a round pen or much space, There is a Kelly Marks (Intelligent Horsemanship) booklet showing how one can win compliance inside the box.
Ask yourself how you put the horse back in its stable. Do you circle round outside the horse and turn to close the door? I think that is BHS way. I now do that because it involves my coming between the horse and its food. But it has to be done with conviction and with a horse which you have already taught to comply in leading outside the box.
It is hard to explain this writing a post on a forum. I learned by watching lots of clinics. Body language, speed (or slowness) when leading and ones history with that particuar horse all come into it. But Rashid who was my great teacher had one fundamental question for clients. Do you want him to do that? If the answer was yes, then it was OKay. If the answer was No, one had to show the horse it was not OK. Yu need to do something when there is an intended nip. For me that means putting the horse at inconvenience by backing it up and or circling. Maxwell does very similar.
Rashid appears to have moved on from that. He commented that all he had done was teach horses to circle. And it all took a lot of time. My view is that it worked for me, it still works and, as an elderly rider, I have all the time in the world and want to avoid physical conflict. I also need to avoid Ella dragging me as we walk from the hitching rail back to her box where she knows a hay net is waiting. One rides in smooth soled boots and I have fallen twice in the yard recently though not when leading. I dont want to fall when leading. So I probably looks as if the whole thing is problem free, It isnt, For almost everyone it is Problem solving.