There is an application called MCEdit. This can edit the world without actually being on the world, making you able to do basically anything. What you want to do is download it, then you need to navigate to the command block. After finding it, select the button on the left side panel, which should be labelled "Flood Fill". After selecting that, choose the block you want to change it to, which most people would choose air. Then, click your command block and make sure that you have stayed on the flood fill tool, which will then fill the command block with air! You can't disable the command block, but you can get rid of it which will disable the one command. Next time you log back onto your world, you will not be killed repeatedly, or whatever repeating command block you have!