But … would not your advice (although valid) not be considered illegal?
If you are hired for a job and you are not compensated, that job is still technically yours.
It is a fine line and almost a contradiction (since I hide under a table so that they do not throw a boot or an infraction).
Entering and destroying the website or replacing it with a "This guy did not pay me!" The page would be absolutely illegal.
Eliminating the work you did is analogous to claiming an unpaid property. You have "repo" your work.
Keep in mind that I'm talking about changes like, making the website load faster and what not. If the OP had to restore the site from the backup after a hard drive failure or that kind of things that change things (obviously)
A whole moot point now, given that the OP has been updated and has obtained at least partial compensation now.
Just remember, your job is your job until it's compensated. If you make changes to a server or whatever and do not pay for those changes, it is completely within your rights to eliminate that work.