Prolly time to put more thought into more effective spam filtering than blocking an entire mail provider.
As of 2011, Hotmail had *three hundred and sixty five million* unique users (monthly). It's not a fly by night email provider, and just nixxing any @hotmail.com emails of users reeks of poor site/spam management tools.
Please rethink this and better your spam management systems. Many people use throwaway email addresses for forums such as A.org, and simply wiping people out over their choice of webmail providers isn't a solution. You can't block 'em all over a few bad apples.
What happens if you knock off all the Hotmail users and spammers with GMail, Yahoo or other similar webmail hosts become a spam issue? Remove any GMail accounts, which GMail has over 400 million unique users (again, monthly)? Put the kibosh on all gmail.com email addresses? C'mon, lol.
Can't condemn an entire domain when your system could possibly be bettered. Banning an entire webmail host is ludicrous in this day and age, and it's virtually unheard of.