How to remove an Hub Transport Server Role from an Exchange Server
When you try to remove a Hub Transport Server Role from an Exchange Server you will need to remove the machine you’re about to uninstall Exchange from the Send Connectors of the Exchange Organization. To do this:
Open Exchange Manangement Console
Expand Organization Configuration
Select Hub Transport
Select Send Connectors tab
Go through and open each of the Send Connectors
Go to the Source Server tab
Select the server you want to uninstall the Hub Transport Role and click Remove (X)
Once this is done you should be able to remove the Hub Transport Role from the server in question.
Zero Day IT - Peter V Nguyen 
Hi,
it’s also necessary (if you migrated exchange 2007 from 2000/2003) to remove the Routing Group Connector thats allow the comunication betwenn exchange 2007 and exchange 2000/2003
To delete in exchange 2007:
show the connector: Get-RoutingGroupConnector
delete: Remove-RoutingGroupConnector -Identity “Source Routing Group showed in the show the connector command”
Now, you can uninstall the hub transport role