You could possibly use the Google Contacts "Import" feature to create a single contact listing all 10000 phone numbers that share that 6-digit prefix. There might be a phone number limit, if so try making 10 contacts with 1000 numbers each (or 100 with 100) - then group them and block the group.
You can see the correct format for a contact with multiple numbers if you create one manually and then export it to a file.
The easiest way to build the list of numbers would probably be with Excel. If you list the numbers 0 through 9999 in column A, you can concatenate them all together in column B, then copy the value from the last row of column B. The column B formula (starting in B2) would be something like this:
=CONCATENATE(B1," ::: 800-555-",A2)