Hi There,
I decided a long time ago that the best way to handle the issue of the client deciding they want things changed is to allow them to decide for themselves that it may not be such a good idea. I have found a very effective way of achieving this.
As soon as we receive a request or instruction from the client that we believe to be detrimental to the SEO for the site, we immediately forward to them a Disclaimer and Acknowledgement Form. The form is accompanied by a request for them to sign and return it so that the requested changes can be completed. Similar to Ninjamarketer's disclaimer, the most important part is that the form carries the words "hereby acknowledge and accept" and requires a signature.
We make no attempt to dissuade the client - just forward the form and require it to be executed and returned before the work can proceed.
It generally takes 5- 10 minutes for the client to call me once the form has been sent and in every case to date, they have decided before I answered the call, that they may not want to make the changes after all
It is at this point that I am able to talk it through with them and do a little gentle SEO "Training" which they are very receptive to. It works for me 
As to partial SEO - we offer on-page services on a per-page basis (minimum 3 pages), but for any project of more than 5 pages, our quotation includes site wide audit, structural review and recommendations for further work. We also provide on-page work via direct access OR as a detailed report which can be handed to the existing developer for action. For clients with limited budgets, this can be helpful as they can attack the work in manageable chunks. We conduct the site assessment and advise them which pages are highest priority for action.
It is nice when you have a client coming back to you for more work because they have seen marked improvement from the original project and want some more of that action 
Hope that helps,
Sha