Just rejecting with a HTML prohibited message sounds so much nicer. Also, how do you propose to deal with quoting? Lots of people like to say "See my response in $colour". All that information will be lost and needs to be retained for context.
rejection is not solution
we can send mail to the user saying that ur message is be modarated and subjected to alteration to convert to text message .as we not encourse html message .so next time post use text format mail .
Regards Sachin Rase