Exhausted the maximum customers

Exhausted the maximum customers

hi there!

got the message today "exhausted the maximum customer".... I once imported my contacts from ZOHO (without actually needing them all in Invoice) and used only about 20 or 30 customers...

Now, when I got the Error-Message today, I thaught about deleting the contacts I don´t use but I can´t delete them Zoho Invoice tells me...

how can I get rid of the unused Zoho-Contacts so that I will have left some more customers to be used fpr sending Invoices to them?

Yours thankfully

Stefan