The way I find out things like this is to go into Excel, record a new Macro, then do whatever it is I want to find out about (in this case format a column to text). Then I open up the Excel VB
Editor and look at the VBA in the macro I just recorded.
In this case you get:
Selection.NumberFormat = "@"
which can be condensed to
Columns("B:B").NumberFormat = "@"