I've done this many times and I have to admit I never actually checked for repeat offenders when I probably should've. I agree with Earthliŋ that giving newcomers the benefit of the doubt is the way to go, not only to cultivate a welcoming community but also because the help center guidelines for asking good, on-topic questions is not obviously accessible from the JLSE homepage or even from the Ask a public question page.
Sure, there's a Helpful links section in the sidebar, but on my laptop screen it barely shows up in the bottom right corner of the monitor so it's easy to miss (see image).
As a language Stack Exchange, it's not unreasonable to think that newcomers believe this could be a place for translations. That combined with the reduced visibility of this website's guidelines makes it unlikely that newcomers will actually notice the off-topic/no translations agreement on JLSE.
Moreover, rather than telling unaware newcomers that they should go to another platform and go through the hassle of potentially making yet another account simply to translate a Japanese image or birthday message, I think giving the translation in a comment is a negligible effort compared to the inconvenience put on the new user otherwise.