Update: It turns out Glenn Beck was joking*. Muse never contacted him to stop endorsing them. Here's a statement from Christopher Balfe, the COO of Mercury Radio Arts:
After raving about Muse for 4 minutes, Glenn made a joke about their representatives emailing him to stop. While it is entirely possible that Muse does not like having Glenn as a fan, he was making a joke and their representatives never reached out to him.
British Radiohead-enthusiasts Muse have an unlikely fan: Conservative hack and professional asshole Glenn Beck. It seems Beck totes loves the bros in Muse, so much so that he devoted time on his radio show to pimp the band's new CD, The Resistance. But it seems Muse don't want Beck's endorsement; they recently asked him to retract his endorsement of the band, obviously because he is the worst. You can hear Glenn's bloviating appraisal of the band as some kind of gods amongst mortals below. The last 30 seconds is where he "jokes" about Muse sending him a cease and desist letter. [Revival via Stereogum]
*- Because Glenn Beck is a comedian, obviously.