Try this, copy your wife's Mii onto the wii remote, delete the Mii on the console and then reintroduce your wife's Mii on the wii remote back onto the console.
If I remember the game/console manuals correctly, it said the Mii's on the wii remote don't retain game save information. And also, I remember when I deleted one of my Mii's, it said something to the effect of "deleting this Mii will remove everything associated with it".
The thing is, Wii Sports will treat the reintroduced Mii as a totally new Mii with all experience levels set to 0.
Now if you only want the points for Boxing down to 0 and everything else as it is, I suggest intentionally losing at Boxing on her Mii a number of times until it bottoms out.
If MaxCapacitor's idea doesn't work, then you could always deliberately just stand there and lose the match a few times to lower the skill level to the desired amount.