I actually think I've found the solution.
create a firewall rule that blocks everything, but deactivate it:
New-NetFirewallRule -DisplayName "Teams.exe" -Program "%LocalAppData%\Microsoft\Teams\current\Teams.exe" -Profile Domain,Private,Public -Description "Teams.exe" -Group "Teams" -Direction Inbound -Protocol TCP -Action Block -Enabled false -EdgeTraversalPolicy Block
New-NetFirewallRule -DisplayName "Teams.exe" -Program "%LocalAppData%\Microsoft\Teams\current\Teams.exe" -Profile Domain,Private,Public -Description "Teams.exe" -Group "Teams" -Direction Inbound -Protocol UDP -Action Block -Enabled false -EdgeTraversalPolicy Block
ps: unbelievable what an administrator has to come up with because Microsoft is too stupid to offer a clean software solution :(