I fiddled around with this idea a year ago, but I can't remember how well it worked. The idea is to have a filament that is slightly larger than the hole in the heater barrel. In the picture I greatly exaggerated the overlap between the filament and the hole. This idea causes the filament to compress and create pressure on the bore wall preventing any plastic from leaking past the filament. This also will increase the amount of force needed to extrude the plastic. Here is the picture:
A better option is to only have a short section that is the small diameter, and then the boar opens back up to relieve the pressure. Here is a picture of that:
I'll try to get around to actually doing this soon. I'll let you all know how it works.