Let's say that someone got a level 100 shiny metacross from a trade with all of its evs maxed out and it's not the way I want it. Can you use a reset bag to completely get rid of its evs as if it never had any?
Yep, the rest bag will reset all EVs of 1 Pokemon to 0.
It'll also tell you where the EVs were, so if you didn't know what that Metagross had, you can save just before you use the bag, and if it's already EV trained to what you want it to be, you can just close the game & keep its EVs.
Yes, that's exactly what the reset bag is for.
Source: My own Reset bag