Sr front end developer misbehaving with my junior developer- what am I supposed to do?

I work at a very lean remote only startup. We don’t have an hr.
There’s a sr front end developer- who tends to misbehave with others. He did it with me once, I immediately shut him down.

But, there’s a junior developer under me (2 month’s experience). The sr developer Is misbehaving with him, the junior complained to me- about it.

What do I do?