Neither Boot Camp Assistant nor Disk Utility can repartition my Fusion drive

After playing around with Boot Camp, trying to install Windows 8.1 on my Retina iMac with Fusion drive (it didn't work), I'm left with a partition scheme that neither Boot Camp Assistant nor Disk Utility can fix.

Boot Camp Assistant gives me the warning:

The startup disk cannot be partitioned or restored to a single partition. The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows.

And here's my partition scheme from Disk Utility:

disk1s5 and disk1s7 have been left over by the Windows installer. Except clicking [-] on either of them doesn't work - after confirming that I indeed want to remove the partition, you can see "Preparing to remove partitions" for a split second, then it just stops. I can't even create a new partition in the free space at the bottom.

Can anyone help? At this point I'm ready to just reinstall OS X and restore from Time Machine, if I knew that that restores the partition scheme. But even the OS X installer usually only formats the Macintosh HD partition.