I've had success (different game) in the past with this guide (the Steam app ID for TF2 is 440
): http://forums.steampowered.com/forums/showthread.php?t=2470784
The basic idea is to:
- mess up (i.e. empty) the local backup data
- start Steam and receive a Sync conflict dialog
- delete the local backup data without closing the dialog
- resolve the conflict in the dialog by uploading your local (erased) data
I have no Mac experience with it, buy the only Windows-specific details there are the use of PowerShell and the install paths.
For step I.2, you can use this (Linux tested) shell command to empty all files in the folder (the install folder is assumed to be correct, but I can't test on Mac to confirm), replacing steam ID with you own (there should be only one folder):
cd "~/Library/Application Support/Steam/userdata/<steam ID>/440/remote"
ls * | xargs -n 1 -I {} sh -c "echo '' > {}"