Restrict to one value per row

I'm not sure if this is possible.

For example,

I have four columns and rows that I'm working with. Each row should only have one value in each columns. ie. A2 and B2 can't both contain values. I'd like only to have one value per row. Is there a way to set a macro that will automatically delete the ..say B2 if there is already a value in A2?

I hope this make sense.

Heather