Edit|Clear|Contents from the user interface.
Or from VBA:
activesheet.range("a1").clearcontents
But if your cell contains a formula, you can give the appearance of a blank
cell, but it won't be blank:
=if(a1>7,"",a1)
(in B1)
But it'll flunk this test:
=isblank(B1)
But you could check it to see if it looks blank:
=if(b1="","looks empty","I can see something")
or if you're used to entering spaces to clear the cell (a bad idea in most
cases):
=if(trim(b1)="","looks empty","nope")