Citrix article CTX126382 describes how to remove a XenServer Slave from a pool, however it does not completely clean up after the process is complete. While the host will be removed, any storage repositiories will be left behind, such as DVD and local storage.
To clean these up perform the following:
1) Click on the disconnected storage repository on the console
2) On the general tab, right-click on the UUID and select copy
3) On the Pool master console, type: xe sr-forget uuid= (and then right-click paste which will insert the UUID of the disconnected storage repository)
Repete this process for all disconnected storage repositories, which is tpically local storage, DVD, and removable storage.