You should not answer questions that you know are off topic, and you should vote to close them.
The policy on these site is that we accept some questions but not others, and the rule is that we close questions that we do not accept so that they cannot be answered. Among other things, this serves to ensure that other readers of the site will only find on-topic questions, and to ensure that we do not incentivize people to ask off-topic questions.
We should be welcoming to new users, as you say, but that does not exempt them from following site policy. What it does mean is that we should make an extra effort to help them understand how the site works, including the rules that apply to their posts.