Sinsull, a good approach is to wedge some towel between the rod(s)/ dowel that connect the neck to the pot and the head. There are also mutes that can go against the bridge.

I still couldn't dampen mine enough for late night playing in the flat. I think one of the reasons is the banjo has the sort of sound that can penetrate and still be heard even if quiet, at least if it stil to sound like a banjo (if that makes any sense).

Jon