This is exactly one of the things protection is meant for. Protecting the question means that if someone wants to post a link to promote their printing service, they have to post a good question or good answer elsewhere first. But blatant spammers tend to have a "quantity over quality" mentality; they want to plaster their links as many places as possible without investing time and effort, since that's how they make a profit. It's not worth their time to come up with another good question or answer, post it, and wait to accumulate 10 rep.
If someone who does have 10 rep on this site posts an answer that seems to be purely promotional, well, perhaps they are just recommending a service that they've found to be useful. Perhaps not. But they do have at least a slight posting history that you can use to help you make that judgement.
Another thing you can do is (get a moderator to) add the "insufficient explanation" post notice, which says that answerers are expected to include some detail in their answers, not just post a link. It also needs to be enforced through aggressive flagging and deletion of link-only answers. If that happens, it further raises the bar for spammers. I think your spam problem would pretty much disappear at that point.
Honestly, though, I wonder whether questions like this, which can be effectively answered with just a link, are really a good idea on SE. The vote totals on answers are rather meaningless for such questions. But that's a separate issue.
