Our experiences from the Dutch ISP market indicate otherwise, customers are more than happy to be informed they might have been infected by a virus/worm. Most customers are too afraid of loosing valuable documents due to a file-eating virus for example, or afraid of loosing connection to the internet entirely and appreciate it to get an opportunity to do some clean-up when placed in quarantaine vlan. They even will recommend you, and your reputation as ISP-with-clue will increase.
To stay on-topic, this is one of the first steps to prevent hosts in your network attacking NATO and decrease the risk of being disconnected by them.
Commercial products that might assist you:
Unfortunately, here in the US, as someone who decrapifies computers for several home and business users, I find that no matter how much I alert users to infections, they just don't care.
"But I can still use my computer! You're just trying to get more money out of me."
You warn them that opening attachments is dangerous.
"But I got this great power point presentation that shows me how to make cookies on the hood of my car, which I would have never seen had I listened to you!"
You warn them that the screen saver they just downloaded and ran sent their passwords and credit cards to a cracker.
"Oh, but my credit card company won't hold me liable, so it's not a big deal."
They install MyCleanPC or similar, which proceeds to install more crapware which eventually starts randomly deleting important files on their computer.
"But I saw it on TV, and people were saying its a great product that makes my 386 perform like a Core i7! Your a computer expert, I'm sure you've backed up my files on your computer without me needed to tell you."
Yeah, things may be different overseas, but here in the US, ignorance is bliss and endorsed by the GOP and Tea Party. Here, people take pride in being the dumbest moron on the block.
In all cases of the above, I was told almost that exact statement by a customer. They will do _anything_ to try and avoid responsibility for their behavior.