For far too long have I been irritated by this pop up every time I open the game client. It has been posted about before by myself and others, and I'm sure it wouldn't be too difficult for WGT to add a 'Never Show Again' checkbox to give us the choice of not having to see it, but they haven't.
I have (by accident) found the file responsible for this pop up, and if you add it to your Ad Block program, you can stop it from appearing.
I made a post about adding things to AdBlock's custom filter list a while back, HERE, so hopefully you can use that to add this file (in bold below) to block the Country Club pop up.
http://media5.wgt.com/versions/*/uiassets/uix/cc/cc_interaction_configuration.json
Good luck. : )
P.S. I apologise to all the CC owners whose clubs I accidently joined due to this pop up. But it won't be happening anymore, thankfully.
EDIT : Sadly, it looks as though this file is connected with editing your equipment, as it takes a while, after clicking any of the equipment icons, for the inventory pop up to appear. You can edit your equipment via your profile page though and the delay doesn't occur if you do it that way.