isolating DC from dmx signal
One option could be to insert a capacitor between the dmx controller data+ pin3 terminal and the signal wire. A capacitor filters out the 24V DC supply completely so that it can not harm the controller. At the same time, the AC signal should pass the capacitor. (To learn more about this, ask google for "first order RC high pass filter", e.g. here). 

But it would be a bit of engineering to get the value of that capacitor right. It depends on the bit rate and also the impedances of the controller and receiver. You might want to start with a 1uF capacitor rated for 50V.