Yanis,
That particular brand of junction block is not designed to come apart easily but it can be taken apart. I couldn't really tell from the photo but I assume that you have a single cable going into one end. First, take the single cable loose at the throttle. Then, If you grip the junction block at the end with two or three cables with vise grips, you can twist out the end with the single cable without twisting the internal cable block. Then you can grip the black part of the end with multiple cables and twist off the red body. Be careful not to grip the red part too tightly except at the very end where the black plastic cap is inside or you might bend it. Once this is done, you can slide the cable housing and junction block parts right down the throttle cable. The difficult part is getting the junction block apart. changing the cables is easy. If you go to the California Power Systems website, they have a much better junction block for about $15 US. The ends are threaded, made of metal, and can be easily taken apart with a wrench.
Bill.