<p>It's pretty manageable right now due to the low question rate, but I think maybe ~3% of all questions this site will get, forever, will be "what's the best printer" or "what printer should I buy" type questions. They're mostly coming from people who don't know enough about 3DP to articulate their requirements, so they're difficult to help and very unfocused. Is there a better way to handle this than locking them as they come up? </p>

I think the site tries to prevent this as best as possible, short of having a bot prevent the post.
There is the
Don't Ask
page and the
On-Topic
(which we have control over its content).
The suggested/related questions that appear when asking a question. Ideally users at least regard its existence before posting the question.
There appears to be a pop-up that does try to warn the user that their question is "subjective". I tested it really quick, see the image below
All else fails, that's what moderators are for. Moderators should utilize the tools available to them. So even if these types of questions pile up, there are means of quickly identifying. A simple feature, available to everyone, is advanced searches. I try to use searches like
this
to quickly identify large groups of questions that may need attention.
It might be nice to have more options over whether keywords can be contained or not, "asker" reputation, "answerer" reputation, etc.