A script is possible, but may cause conflicts with other scripts e.g. "themes".

If it's only one user/bot here and there, I suggest using the ignorelist instead: the item "[control] codes" allows you to "strip" (ignore) all control codes (colors, underline etc) of a specific usermask - try: /ignore -k NickOfTheBot
Alternatively, you can access the ignorelist in mIRCs address book (Alt-B), at "control" select "ignore", click "add", put a usermask like Botsnick!*@* and in the "ignore options", check the "control" box.