Will you ruin your iron if you use it to wax your snowboard...?

You would not be able to use it for clothes again. I bough an iron from Walmart for $15 to use on my skis. You don't need a special waxing iron unless you are using some very specialized and expensive waxes. The steam holes won't hurt the board. As for the temperature, if the wax melts easily on the iron, it is hot enough. If the wax smokes, it is too hot.

You'll never get it clean of the wax so if you want to use a household iron get a cheap on and use a real LOW setting. Ski wax irons are really cheap these days you can find them in stores for under $40 and not worry about ruining your skis or your clothes

You can, but IF (big if) you do use a household iron, it needs to be one without the holes for steam. The bottom should be entirely flat.

You will ruin your iron and possibly your snowboard. Ski and snowboard bases are very temperature sensitive, and for this reason irons designed specifically for waxing will have a very precise thermostat which only fluctuate a few degrees in each direction. Household clothes irons will fluctuate up to 50 degrees fahrenheit in each direction.