Downgrade El Capitan Developer Preview to Yosemite
If you're looking for a Terminal solution, you can try this:

diskutil eraseVolume free %noformat% disk0s2


or 

diskutil eraseDisk free %noformat% GPT disk0


In this case, disk0 should be changed to match whatever your actual device node is. It will depend on whether you're using a USB stick to boot into recovery – in which case it's probably disk1 – or if you're actually booting off of the disk you're trying to erase.

And an important disclaimer: If you blow away your disk, you will not be able to boot into the onboard recovery mode. In fact, you probably aren't going to be able to do this without booting off of USB. I hope you have a recovery stick somewhere, or another computer from which you can make one.

It should go without saying that you will lose data if you run either one of these commands successfully.