There's code in the Mac app to automatically expire clients that haven't synced recently. I don't know if that code is also in the iPad and iPhone clients. Email the
omnifocus@omnigroup.com for a definitive answer.
You can manipulate the client list directly by mounting the Omni Sync Server directory on your Mac or PC, then opening up the OmniFocus.ofocus package file. There will be files ending in .client for each sync client, and if you open one up you can see which client it belongs to, when it last synced, etc. Simply delete that file to remove the client from play, but be aware that if there are unsynced changes on that client, they will be lost.
On the Mac, you can mount the Omni Sync Server directory from the Finder with Go->Connect to Server... and using https://sync.omnigroup.com/<username>/ as the path, where <username> is your OSS username. If you are using Snow Leopard or earlier, you might have to try sync1.omnigroup.com, sync2, sync3, etc. until you find the one that works, thanks to an Apple bug.