So, what are your advices on preventing a client to cancel a contract and simply keep the software installed in the server (cut my ssh access, etc. and keep the site running).

If that is your main concern, I suggest hosting the site yourself. If you can't or don't want to host the entire site, then maybe you can make your product into an RPC-type server (client makes remote calls to your server, which processes the information and returns results).

I'm sorry if this isn't an ideal solution, but in my mind it's the only real way to keep your product safe in a situation like that.

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