I would just move on if they do not owe money. In my contracts I allow them to cancel with a 30 day notice. However if I have applied fertilizer, seed, did any lawn spraying, trim shrubs, mulch pine needles and anything other than mowing I did extra or spent money for, then they have to pay for the (extras) and also account has to be current with up to date invoices paid before I let them out. If they have to pay for the extras before they get out of contract then that helps keep them under contract or your guaranteed to get money that you spent on the client.
Hope this works!