Sir it is not possible to unslave the slave server from Slave web panel.
But you can unlsave it from command line by modifying the files.
But we do not recommned it as the entries of this slave will be there on Master server and when your Master will come online it will cause issue when you add try to add this Slave server again.
To unslave the slave server from command line please edit the following file :
/usr/local/virtualizor/universal.php