Contracts made under duress are unenforceable. For a good overview, go to the link below:
Contact the agent's broker and tell them so and that you consider the contract made under duress caused by the agent. If they push back, don't argue, just get a competent real estate attorney to send a letter to the broker. And if you have to go to this extent, then also consider filing an ethics complaint.
In the future, when confronted by that sort of situation, ask yourself: "Is this the sort of person I want representing me, talking on the phone to possible buyers or other agents? Is this the sort of person I'd want to deal with when I have further questions?" Your answer, of course, is no. And that makes it a lot easier to stand up under the pressure. In fact, the more pressure that's applied, the stronger your internal "no" becomes, and thus easier to verbalize it.
Hope that helps.
If this agent works for a firm he is supervised by a broker. Talk to that person first and let them know how you feel. If you don't get satisfaction from the broker then call the state department of Real Estate and ask them how to proceed.
If those efforts are unsuccessful then contact an attorney on how to get the contract voided. In the meantime don't let anyone pressure you into signing anything else.