Well, a starting point might be the CurrentRegion property of the
selection object. You could do it like this:

Range("AA1").CurrentRegion.Select

Try it and see the result  you get. It's pretty good. You might need to
restructure your sheet a bit to sort of put  a moat around the data. Try
it out and you'll see what I mean.

Once that's selected, it's pretty straightforward to look at every cell
in the selecion in turn and evaluate it's value. It would be something
like this:

strCheck = Range("A3").value

For each c in Selection
If c.Value <> strCheck Then c.Formula=""
Next c