Agents are in real estate to list and sell homes....to take a position of not being willing to do so is difficult to imagine.
If you feel an agent has behaved unethically and/or done you wrong, probably the first place to begin is by contacting their broker and expressing your concern.
Remember that agents don't sell homes...sellers do.... If you did not get a property you bid on, there could be loads of reasons for that...
Realtors can and do choose who they want to represent...I've turned down business based on whether I think I can get a client what he or she desires...
Out and out blackballing is of course illegal....I just think this would be a very hard thing to prove.
Good luck to you.
I'll contact the agent from porterville below and see if this is my imagination of not being able to buy in porterville.