Plusnet's newsgroup access is an outsourced service supplied by the popular Usenet provider Giganews. It is generally accepted that Giganews' spam filters aren't particularly aggressive when compared to other Usenet providers so you may want to take additional measures to reduce the amount of spam that's visible in the newsgroups you frequent. Nowadays a large proportion of the spam messages found in newsgroups originate from Google Groups. Further reading supporting this claim can be found here. One way to vastly reduce the volume of spam you see is to filter out all the messages that have been posted using Google Groups. Here's one way to do it (in Windows) using a lightweight application called 'NewsProxy'.
Create a directory called 'NewsProxy' in your Program Files folder and copy the contents of the zipped archive across to it.
Create a shortcut to NewsProxy.exe and copy this to your startup folder (you should just be able to drag it to your Start button > Programs > Startup folder). This will ensure it's run when your machine is switched on.
Create a new file called nfilter.dat and save it into the NewsProxy directory. This file should contain the following line of text (including the asterisks):
* drop Message-ID:*googlegroups*
Open NewsProxy and go to Edit > Configuration.
On the General tab, select Hide on Startup.
Select the Network tab, change the Server name to news.plus.net and click OK.
Open your news client and create a *new* news account but use the server address 127.0.0.1 instead of news.plus.net.
Exit your news client.
Reopen your news client.
Refresh the list of newsgroups for the new account and resubscribe to all of your groups.
Recreating your account and renewing all of your subscriptions is a pain but the above steps will completely eliminate the spam that's posted from Google Groups.