Sometime you come across situation where host has not been ejected from the pool master. This host has been shutdown and decommissioned. In such situation stale host will not get out of XenCenter .
To remove ,try following steps
This needs to be run from windows box where xencenter is installed
xe –s <Pool Master ip address > –u root –pw <password of root pool master >
This will give UUID of stale host as shown below
Now run xe 0s <IP address of pool master > –u root –pw host –forget uuid=<stale host >