Just but the iPad in Dfu mode. Hold down the home and power button for 15 seconds connected to the iTunes. iTunes will detect an iPad in recovery mode. Restore the iPad this will delete all the content in the iPad making it like a brand new device. Once it's finished you can sign in with your apples and use iCloud etc.
I'm having the same problem..And don't know what to do
I sympathize with you. I'm in the same predicament right now. The person I purchased my iPad from won't respond to my queries. Everyone online will act like you are a thief. If half of them used their brains for a minute, maybe they could come up with one of many scenarios where you would be legitimately locked out of an iPad you legally purchased. In my case, the previous owner left all their personal items on the iPad and I went to erase all contents. It asked me for the Apple ID, I figured it was asking to return checked out books or something. Second time it didn't ask me and reformatted the iPad, but now I'm stuck at activation.
To answer JimHdk's question, Apple doesn't even show you the full email address. It's literally shown as Semail@example.com so you can't email that person.
This is a horrible idea on Apple's part. They should have made it painstakingly clear that you can't even "reformat" and activate the iPad without this ID. Especially since iOS7 is relatively young and no previous version of iOS had this feature. This will hurt legitimate users more than thieves. Even if, from all the people affected, only 10% are legitimate users, that's still a huge number. Apple should give us some other way to resolve this issue.