On a Stack Exchange site I moderate, there is one user who repeatedly uses commas instead of other proper punctuation marks in posts, and often writes simple walls of text. At first, people edited the posts and left comments telling the user what their errors were, but the user got upset and stated that they were a non-native English speaker and were unaware of the rules. I suspect they're not used to using the Latin alphabet.
An example of their responses:
Im sorry,To be honest i write like this because i dont know how to use commas in my keyboard.I didnt reject your edition on porpouse,I was adding more information...Sorry
This was fairly tame, but came after they were corrected four times in the same post.
I understand that not being a native speaker can make communicating harder, but they have been told many times in the past what is the right thing to do, and they seem unable to follow the directions. It's adversely affecting how their posts are being received. I want to let them know that they're not writing correctly, without them getting annoyed. I don't think they don't have cause to get annoyed - I think that they're in the wrong - but I still don't want to seem overly antagonistic.
How can I politely tell them that they need to quickly change their behavior, without coming across as insensitive or rude?
I should also mention that this user has had other behavioral issues in the past, including asking poor questions, sockpuppeting, and not following other site rules very well. They have been mod-messaged multiple times and suspended. Making questions clearer is one of their major issues, but they do have a history, to put it kindly.