We do not support hibernating and reactivating myBalsamiq sites. We keep sites with inactive subscriptions for a minimum of 1 month. After 1 month, the site data may be deleted.
If you won't be using myBalsamiq anymore, you can download all your projects to your machine. Make sure you save them somewhere safe in case you need to come back in the future. These projects can be uploaded to Balsamiq Cloud if desired.
On the other hand, if you're already using Balsamiq Cloud, you have the option to allow your Space to auto-hibernate. This still requires that you download your projects locally, but you won't be billed if you follow the steps detailed here: Balsamiq Cloud Auto-Hibernation.