How do you deal with a student who only wants to write on one topic?

Yes I do have one such student who only likes to write on himself.Well this may be pointing towards his lack of perspective upon other things.By giving them interesting topics to read about may divert their fixation on that one topic & will give them vocabulary & interest in writing about other substantial topics.

In fact, I am dealing with myself first, because often the student for up to this because of the way followed by teacher

I'd encourage him to achieve mastery on that topic instead of discouraging him or pressuring him intto giving up his favourite topic. Later, I'd develop his interest in other topics.