Put them on your boots and do up the strap. Tie the strap together with a neat knot about 7cm from the buckle. Cut the strap with a few cm from the knot at an angle. Melt the strap with a lighter to stop it fraying.
Or alternatively tell yourself that at some point you might want to wear these crampons on an ascent of Vinson or Denali in winter and you'll need lots of strap to go around the massive double boots and over boots you'll be wearing, so not cut the strap. Then spend 15 years staring down at your feet when ice climbing watching a floppy bit of strap flap about - not actually doing any harm but sort of annoying all the same - realise you're never going to Denali in winter, then cut off the bit you don't need.
...until you walk out, and the floppy strap is now flopping about under your feet, you stick a crampon point from your other shoe through it, trip yourself up, and slide down a Neve slope to your death, at which point it is actually doing some harm of a particular sort.