Are you using a separate router connected to the Comcast gateway device ?
Log into the admin control panel at http://10.0.0.1 - unless you changed the account credentials, the default login is
then on the left side menu go to
Gateway | Connection | Wireless
and you should be able to turn off 2..4 GHz from there, or, if you wish to try this first, try changing the channel of the 2.4 GHz wireless from Auto to either 1, 6, or 11, before you turn it off completely.