I'm with CJ, if the user is breaking the terms of agreement, i don't think the way to go about it is to cause changes on the site...
Of course if the user switches it on they can always use a little CSS to hide the element instead, and it'll make it just as hard for you to write another script to counter-act that, and so on... you'll end up going in loops..
If you have a contract or agreement with this person, then the sensible, and professional approach would be to write them a letter/email stating they are in violation of the agreement and your intentions if they continue, etc...etc...
Having another read of your post, it would be my understanding that you have been contracted or made an agreement to do some work for someone and you want to ensure your link and ads remain....
If it's a wordpress template i don't think you'll be able to create anything that can't always be removed by the user, unless you have direct access to the WP core files they are using, but then if it's the user's/individual's webspace that's not really within your right to do, regardless of the terms of agreement.
It's shady ground, you can set an agreement but you can't ultimately force it upon someone if they are determined to break it.
This is what contracts are for, so you can make the individual liable for any breach of said contract. Not really sure of the technicalities of contracts/agreements in relation to WP installations though, it's not my field.