You would be very surprised at the creative science that they have to backup many of their claims. It's much more difficult to disprove than you think. I actually do it just for fun, but I thought maybe a good proof for them to see would be people that actually fly and see the curvature of the Earth every day. From my research I gather that you can see the curvature of the Earth at round 50 to 70thousand feet. A lot of people that have flown the Concorde recall seeing a very pronounced curvature especially around the northern hemisphere.

The group is called "Official Flat Earth and Globe Discussion Forum" on Facebook. If you're curious you should give it a try and debate science with these people it's a rather frustrating at times though I must warn you.