If you're on that screen in the installer now and reformatting through the graphical interface isn't working, press Shift-F10 to bring up a command prompt. Type 'diskpart' to launch the command line disk partitioner.
use these commands to navigate:
list disk
list partition
select disk x (where x is the disk number of the target disk as listed by above command)
select partition x (same as above, for partitions)
delete partition
convert gpt
exit
try again