Teve que adicionar o seguinte a /etc/modprobe.d/blacklist.conf
blacklist radeon
Agora, ele fecha o X corretamente, mas recebo os seguintes avisos:
(WW) fglrx: No matching Device section for instance (BusID PCI:0@0:1:1) found
(WW) fglrx: No matching Device section for instance (BusID PCI:0@0:17:0) found
(WW) fglrx: No matching Device section for instance (BusID PCI:0@0:18:0) found
.... etc