If your client is unresponsive to your request, you can end the contract. You can follow the steps shared in this help article. This action will still send a notification to your client requesting them to share their feedback working with you.
We have been told it's best for a freelancer's JSS not to close more than a couple of idle projects at a time, spreading the closure of multiple idle projects over multiple two-week periods.
Of course, no one can say specifically how many is too many or how often is too often, so the best you can do is do the closures over time, not all at once.