I recently posted a question targeted at beginners and I stated so in the question. Shortly after it was asked, a high rep user answered the question. It was an honest mistake, they answered before reading the flavor text explaining it was for low rep users, and they removed their answer until a low rep user could answer. Still, it begs the question, is there a way I can tag a question as "beginner" or "low rep users" to avoid this in the future? If this tag doesn't exist, should it? Personally, I think having a way of quickly finding "intro-to-puzzling" questions would help engage new community members.
A tag like beginner or low-rep-users is what would be considered a meta-tag. Meta-tags are generally frowned up on on Stack Exchange; tags are meant to describe the content of a question as opposed to information about the question.
I think what you did is sufficient, and if you clearly label the question as "beginners only", a tag shouldn't be necessary.
I suggest starting the question with something like this (you can use your own words, of course, or feel free to copy mine):
Low-reputation users only!
This puzzle is intentionally aimed at those who are new to this site and/or those who haven't had much opportunity to participate because experienced users tend to answer questions fairly quickly. If you are an experienced user, please refrain from answering this question.