I have an areas through which the dog wire must pass and which I need to allow the dogs to have unimpeded access, due to the structure of a dock and bulkhead in this area the wire will pass above ground, to reduce the radio signal in only this part of the loop can I run the wire through Electrical Metallic Tubing (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).
I don't know what frequency dog fence runs at, but EMT is pretty unlikely to do much. chris is right in that it will need to be well grounded to have any chance. Another option would be to use normal rg6 coax, in place of the dog fence wire, its advantages are 1 it's designed for this kind of thing, 2 it's cheaper than EMT per foot 3 its radius of curvature is much lower, 4 its hole size at the end is much smaller.