My initial though is this:
First of all, you can ask the clients to close the contract, now that the initial original work you had contracted for has been done. Don't forget to ask for some detailed feedback.
Then, you can ask them to start another contract for the other work.
At this stage, if/when they do this, you might want to re-negotiate your terms - extra money might put them off, or if they agree, it might persuade you to carry on working for them!
(It is also important to realise that in the future these clients may have the type of work that you may want to do.)
OR, after the contract is closed, you might want to politely mention you are not looking to do that kind of work any longer.
My only reason for suggesting that you seek to end the contract first, without discussing the fact you do not want to work for them any longer, is no matter how nice a client is, they may get upset and they may let it affect the feedback they leave for you. It should not, but we are all human.
However, saying all that, it all depends on how long your working relationship was and how well you know your client.
You may be able to discuss all these things together, without the need to not disclose some information.
You will have to decide what to do quite soon, because the longer you leave it and quality of the work you do and/or your response time to their requests will deteriorate - it is only natural when you are not enthusiastic about some jobs.
That will eventually lead to a breakdown of the good relationship you once had and of course you will get terrible feedback when it is all over.
