You should be able to "choke" ( stop the flow) of the brew with the grinder set to its finest.
I suspect you are not grinding fine enough.
Search / read these forums for post re the BCG800 grinder mods.
there is a shim set available from Breville that enables a finer grind setting to be obtained.
If its a new grinder, you could return it to the retailer for an exchange.