I made a world save for this mechanic, so you you can preview it before putting it in your world. It works very simple. You shoot an arrow with the bow, and when it lands in lava, it places an obsidian block! However, you can erase this with your "reset" item. This will remove all blocks that you placed with the bow and fill in the lava. I also have a schematic of this mechanic, so you can paste it in. It has a few glitches, but I will update it when I can.

IMPORTANT: IF YOU USE THIS IN A MAP, YOU NEED TO CREDIT ME!

Now that that was over, wait... why havn't you clicked the download button yet?