The next time you receive a call from them, after you disconnect, dial *60. That should block that particular number. But beware, they change the numbers routinely so you may have to do this ever so often...You can also log into you account, go to "Voice" ( the phone receiver icon on top right of page), review your call log there, and at the right of each log entry, you'll have the option to block that caller also, or it will show you that it is blocked and you could unblock it. You can block up to 25 numbers....and the list fills up fast...
Good luck to you in managing to dodge this pain all of us are dealing with.