Only cells have a format for numbers. Moreover, a format may be assigned to a range (a collection of cells). Set all cells on each worksheet in the workbook to text using a loop through each worksheet. The approach will be similar; you'll only need to specify the complete sheet. This example specifies the Used Range rather than the entire page. That's not hard; I'm simply keeping the example straightforward.
Dim wkshCurrent As Worksheet
For Each wkshCurrent In ActiveWorkbook.Worksheets
wkshCurrent.UsedRange.NumberFormat = "@"