By similar methods that other peace agreements have been enforced at various points in history? Or maybe other methods. It's what the big brains would do well to be thrashing out right now instead of stirring up yet more death and destruction. I'm sure some sort of solution could be found if there was a will. It will take recognition of the reality that there can never be a total victory for any side and, as I keep saying, acceptance of the fact that it will be messy (not as messy as death and destruction however.) And even if temporary, it would buy time for longer-term solutions to be found. Who knows, maybe the Putin regime would be deposed internally in the meantime.