Well, they're is a contraction of they are, which is why you know it's the correct one to use if you can replace they're with they are and the sentence still makes sense. The word their is possessive so if you're talking about someone or even something possessing something else, you would use their. There is in reference to something or somewhere else.
I can't remember the specific rules I was taught in school, but I still know the correct usage many years later.
There was a snake over there, they're trying to find it now, cause it isn't native and none of our friends say it is their snake!
So what happens if the thugs in the DoJ clash with this judicial security? Are they prepared to turn their weapons on the DoJ officers? If not, then this is only to protect judges from stochastic terrorism at the hands of tRump supporters. Dark passages up ahead, I fear.