If this is your product and the above mentioned was listed when the client purchased it then the client should have been aware of what they were purchasing and the policies about said purchase.

In my opinion you do not have any obligations to refund money to them for services they had already used.

Maybe if they somehow did a pre-pay for say a year's worth of operating costs up front based on an estimated usage and then bailed before they used it all maybe there may be some grey area where you owe them a prorated amount of what they didn't use.

Much like say you get a car insurance policy and pay for all 12 months up front, then all of a sudden you get a new car 9 month into it and get reimbursed for the 3 months you didn't use.

In my opinion any services they used they owe your company for.

Services they were planning to use but didn't actually use yet may be a grey area.

If you do not allow them to pre-pay for blocks of use ahead of time this isn't an issue.

Again I'm not a lawyer.