If the attacks are coming from a relatively small IP range, I believe you can put rules in your firewall to block inbound traffic from those addresses.
yes you can, however, you STILL have to absorb the traffic, crippling your line / circuit. Most attacks are designed to consume your bandwidth, firewall rules do nothing for you in this case. It needs to be filtered upstream somewhere.
Other attacks are designed to overload a particular server with a high number of service requests that it cannot handle, those types can be squashed with a firewall rule.... but attackers know this, thats why the go for the bandwidth soaker approach.