I may have unique insight to this problem. My company works solely with Attorneys and Law Firms we have 200+ clients across the country. On occasion a client may get in arrears with their billing. Based on the fact that you have no contract and therefore no enforceable licensing on your work product collecting will be difficult. Your only strong position is, you can prove payment for services over the past 8 years and if they gave you no formal notice of stopping your services and you can demonstrate that you continued to provide services for the period in question then you should be able to make a case for payment.
Since you are not hosting the site, have only FTP access, and have no end user license agreement you cannot sabotage the site by putting up a blank or coming soon page. Your only recourse is to cease providing support services. If they send further support requests, change notices, or updates make them aware that unless the account is brought current they will sit in the queue. However, these requests then become acknowledgment of their professional support relationship with you and need to be documented. Also, all work performed directly requested by the client during the period of non-payment needs to be documented.
All attorneys and law firms are bound by law to a strict code of ethics that are stridently enforced by their state bar association and in some cases the state supreme court. They may not fear you taking them to small claims court for $860 but they are extremely afraid of being in violation of the Bar Rules or having a complaint lodged against them with the Bar Association.
If I were in your place I would send the Partner the following:
For the past 8 years I have worked diligently to maintain and support your website at www.XXXX.com. During this period we, as partners in the effort, met our commitments to each other. Six months ago, without notice, you ceased paying the agreed upon rate for the services provided. As not notice to cease the services continued to be provided. Our regular services along with requests from your company to make the changes and/or updates were the following:
The request were executed and the site updated.
I value your business and certainly want to continue providing the best professional services possible, which is why, effective immediately until the account is brought current I will cease all support, and take no action, until the outstanding balance is brought current. If further refusal to pay occurs I will be force to take action to collect. Action may include but is not limited to seeking a judgment in small claims court to notifying and lodging a complaint with the State Bar Association.
Unfortunately and only after repeated unanswered attempts contact you about this am I forced to take this action. Please contact me at XXX-XXX-XXXX to resolve this matter today.
Get a contract that licenses your work so if payment is not made you can deactivate your work. If you are fee for service get a guaranteed term and guaranteed form of payment, direct bill to CC or ACH. If you are a month to month provider stop servicing immediately on the 5 day after billing if payment is not received.
Final Comment, If you stopped paying this attorney or your retainer was exhausted do you think they would do any free legal work for you, of course not. Why would you extend more courtesy to them then they would to you?