Firstly, if you are not aware, mind your own business is an aggressive, often confrontational, comment. As mentioned in other comments, MYOB is really only used to respond to unwelcome inquiries about your private life (and even then only certain inquiries from certain people). Outside of that usage, telling people to MYOB will often result in them taking offense to your comment.
You cannot respond to minor physical intrusions by saying MYOB. Doing something like that would be seen as unacceptably aggressive and could escalate into a confrontation. In your example, person A would first have to verbally engage person B before responding that way.
SCENARIO 1 - no verbal exchange before the MYOB - confrontational usage
This scenario is highly unlikely to occur because MYOB is generally not used this way.
Person B shows up and starts rearranging chairs.
Person A: Mind your own business.
Person B: You talking to me? Why don't you f*** off a****le.
SCENARIO 2 - verbal confirmation before the MYOB - inappropriate usage
Person B shows up and starts rearranging chairs.
Person A: What are you doing?
Person B: I thought you needed some help with the chairs?
Person A (civil response): I'm very picky about how the chairs are arranged. I kind of like to do it myself. [In this situation Person A and B remain on speaking terms and Person A still gets to arrange the chairs.]
Person A (uncivil response): Well why don't you mind your own business. [In this situation Person B throws away person A's lunch several times over the next year.]
SCENARIO 3 - verbal confirmation before the MYOB - acceptable usage
Person B shows up and starts rearranging chairs.
Person A: What are you doing?
Person B: Becky told me alcoholism runs in your family so I figured you were probably hung-over this morning and needed some help.
Person A: Do me a favor and mind your own business. I'm fine here.
