From the writeup:
W32.Blastclan.B is a worm that spreads by copying itself to network shares.Basically what is happening there is that you have a machine that is trying to copy the threat from an unprotected machine to the protected machines. That is why Symantec Antivirus is detecting it but it comes back again. You need to find that unprotected machine to stop the infection.
It is quite likely a laptop that somebody plugged into the network or an "under the desk" machine that is managed by your IT and therefore it may not have antivirus or out of date definitions.
There is a feature called threat tracer in SAV that tells you from which machine is the threat coming. You can also see the open connections with a netstat or tcpview from sysinternals.
This should solve your issue