How can I make a regular corner punch from a fancy one?
If you are just trying to remove the hearts feature, or any feature for that matter, from the punch then you would have more luck disassembling the punch and cutting off those portions. I don't know how your particular one comes apart but some are designed to be reassembled. Consider the following one where you are able to remove the cutter:

 

What you could do now is use a hacksaw or rotary tools to remove the areas that you do not need. Vises or clamps can help keep the cutter stationary while you are cutting. Be safe and use eye protection when doing metal work. 

You might risk small indentations from the heart recesses but I don't think, if it does happen, that it will be significant. 

I suppose you could just use a rotary tool while the cutter is depressed but that would be more difficult and would damage the surface of the punch as well.