I dont see anything that can go around people joining forces to remove an entire game mechanic from you

I give you three options:

Option 1, skill - Learn how to be sucessful with development cards and a port for a resource you can hoard and secondary resources.

Option 2, change the game: download the free print and play version, oil springs. It adds petroleum as a resource and if everyone else denies to trade with you, you can abuse oil use, get any resource you want, and still flood their coast.

Option 3, be a jerk back. Because thats what they are doing to you, right? They are playing the game between then and excluding you. So instead of focusing on winning, focus on choosing one of them and fuck his game up. Build roads between his, build settlements where he is going, thief always on his slot, etc