Thus we can probably assume that there isn`t a quick fix available for this problem.
or they just don't want to look for it? civilization that developed interstellar travel couldn't protect itself from simple and weak electro-magnetic field?
Well, I am not saying that being a bit stupid is forbidden from advanced civilizations, but it certainly doesn’t help when your objective is to kill every thinking creature living in the galaxy. To achieve their goal the Trith will probably have to risk the lives of every member of their species, and use considerable amounts of resources to build warships and to research military technology. And if they are prepared to do all of this, even though there could be a quick fix available to their problem, they are in my opinion, either a bit stupid or just crazy.
The above is based on the idea that telepathy is based on electromagnetic fields... This totally precludes telepathy from being involved with species that Don't use electrical or magnetic means of processing information, and also raises issues as above.
If there is "telepathy" in the game, more than likely there would be several types. The Trith would have a very unusual type (probably similar in some ways to George's) in that it travels interstellar distances at apparently FTL speeds. This implies that there is some physics at work that is not explainable by 21st century earth science (as there is in the fundamental travel mechanism in the game)