Within the Sales Ledger Account on the Terms tab you can set the status of an account to suspended.
This will stop invoices being raised against the Sales Leger account.
If the status of the account is set to Supsended (Auto) this is being driven by the Auto Stop (days). This can also be found on the Terms tab.
Auto Stop (days) - if an invoice has not been paid after the number of days set for auto stop then the status of the account will automatically change to Suspended (Auto) and will not let invoices be raised against the Sales Ledger account.
If you wish to change the status back to open from Suspended (Auto), you will need to amend the days set within the Auto Stop days on the account, otherwise it will automatically revert back to Suspended (auto).