Can I run a radio dog fence loop through EMT to reduce the signal?
To shield the surrounding area from an RF source such as an "invisible" dog fence, you must enclose the RF source in a very good conductor that is well grounded.  EMT may not be good enough (being zinc plated steel) and it will certainly have to be well grounded to have a possibility of working.

Another way to do this, which I have done myself, is use twisted pair wire for carrying the the signal.  In those areas where you want no barrier run the twisted pair together and in those areas where you do want a barrier separate the twisted pair into an open loop.  To be invisible to the dog's receiver, the twisted pair needs to carry the outgoing and returning signal so that they will exactly cancel, and to be visible to the collar the conductors need to be separated into a loop by a foot or more (i.e. either along the top and bottom of a fence or at ground level).