However, the diskpart command-line utility can be used to delete this partition. By the use of the clean command all partition information will be discarded on the selected disk.
WARNING: this will not only delete the EFI partition but all partitions and everything on the disk.
select disk <number>
After that you’ll be able to partition the disk in the Disk Management GUI. Or you can do this with diskpart, too:
See the Technet Documentation for more infomation on diskpart.