It can be done, through a client-side mutator.
It's not very hard I believe, but requires a fair understanding of replication back and forth.
However, although I understand why you would want to do this (many games have similar issues), I wonder if that would be effective at all.
Many players (including me) increase the sensitivity of the mouse in-game for legit reasons (it's generally too slow), plus I don't know how UT controls sensitivity, as in: if changing the mouse sensitivity outside of UT influences the mouse speed in UT (I never made this experiment).
A good game will control the sensitivity by actually controlling the mouse speed and preventing it to go any faster than it should, a not so good game will multiply the sensitivity by what is already set in the mouse, and I am not aware which one does UT use.
Still, if you want to pursue this, rather than an exact value, you should have a maximum value instead which you think is reasonable, and allow everything below it.