help with what to do against amazon.com

Hi, I get paid commissions from amazon.com for referring customers through google adwords. A few days ago they took away $1000 from my account because they said that I was advertising using the keyword amazon or a variation of it. It states in their agreement that you cannot use amazon as a keyword, but my keyword was not amazon. I have written emails to them explaining why I should get my money back, but they just told me they would not respond to further emails. My advertisement was showing up when people also included the keyword amazon in their search which is why they were upset with me, but my keyword was not amazon. Their agreement states that once they ask you to to add amazon as a negative keyword, if you do not do so they can take back your commissions, but I complied the very same day I got their email. Any help would be appreciated. Should I try to sue them?

Thanks

Here is their written agreement that talks about my situation.

"you may not seek to purchase or register any keywords, search terms or other identifiers that include the word "amazon", "kindle" or variations thereof (for example "ammazon", "amaozn", "kindel" etc.) ("Proprietary Terms") for use in any search engine, portal, sponsored advertising service or other search or referral service. From time to time we may request that you cause any applicable Web search provider to exclude Proprietary Terms from keywords used to display your advertising content in association with search results, assuming the provider of such Web search engine offers such exclusion capabilities. If we determine, in our sole discretion, that you have engaged in any of the foregoing activities or, as the case may be, refused promptly to comply with a request from us to exclude Proprietary Terms from any keyword as mentioned above, we may (without limiting any other rights or remedies available to us) withhold any referral fees otherwise payable to you under this Agreement and/or terminate this Agreement. "