the exact meaning of barring

[kjc0123]

What is the exact meaning of barring in the following sentences? Does it mean "if there is not something" or "except something" or anything else? Let me know the exact meaning of barring. And if possible, let me know why it comes to be used such meaning.

 

"Indeed. Your key is like a Swiss numbered account, which are often willed through generations. On our gold accounts, the shortest safety-deposit box lease is fifty years. Paid in advance. So we see plenty of family turnover."
Langdon stared. "Did you say fifty years?"
"At a minimum," their host replied. "Of course, you can purchase much longer leases, but barring further arrangements, if there is no activity on an account for fifty years, the contents of that safe-deposit box are automatically destroyed.

[jno]

to 'bar' is to prevent (as in the word 'barrier') or to rule out. So it means 'If we rule out the possibility of further arrangements for your account' or 'Unless you make some other arrangement for your account'.