We thought we had language in the UAL contract that would accomplish just that. For instance, express cannot fly between hubs. Why not stipulate the city pairs, or just say express may not fly any route that was previously flown by mainline without consent? Or mileage? There are any number of ways to write this into a contract...

For example...

1-C-1-c Feeder Flying on Company Routes
1-C-1-c-(1) A Feeder Carrier shall not initiate a new
scheduled Feeder Flying Round Trip in any Market
operated by the Company at any time in the preceding
twenty-four (24) months, unless the Company
demonstrates that a Company Round Trip that may be
initiated in the Market instead of the Feeder Flying
Round Trip would not pass the BIRR Test.
1-C-1-c-(2) The Company shall not remove a
scheduled Company Round Trip from any Market
served by Feeder Flying unless the Company
demonstrates that the Round Trip to be removed would
not pass the BIRR Test in the absence of a Feeder
Flying Round Trip scheduled to depart within thirty (30)
minutes of the Company Round Trip.

Remove a sentence or two and all is good...