If you search most any forum (e.g. AutoDesk or cadalog.com) you'll find this topic thousands of times - must be an epidemic problem.
'Change' command-works on some items but not lines with differing Z values
Express Tools -'Flatten'-About the best I suppose
'Flatten.lsp' freeware, many floating around out there-variable success rate
From the command line - "move" (select objcts), From: 0,0,0 To: 0,0,1e99; then Move: previous From: 0,0,0 To: 0,0,-1e99. Works on simple objects but not nested blockks, etc.
HTH,
Carl