You could also assign the entire range at once since every cell is
getting the same value. Change A65536 to whatever is your longest
column.

Sub Test()
ActiveSheet.Range("J2","J"&Range("A65536").End(xlu p).row).Value=0
End Sub

Alternately, consider assigning the last row number to a variable to
make your code easier to comprehend, especially if you need to
references it in several places; like so:

Sub Test()
Lastrow = Range("A65536").End(xlup).row
ActiveSheet.Range("J2","J"&Lastrow).Value=0
End Sub