a few assumptions here...

the protected data is contiguous - i.e. the blank rows come at the end not
in between.. if this is the case then the following will help

insert a range like below. the example uses Sheet1 and the protected columns
will be H & I & J. This range expands as data is inserted into column H.

=OFFSET(Sheet1!$H$1,0,0,COUNTA(Sheet1!$H:$H),3)

unprotect all the cells on the sheet

give the sheet a password - in the example it is "mypassword"

insert the following code in a module - it runs each time the sheet is
opened and protects all the data in the range - in this case data in columns
H, I and J
the rows below the last entry in column H are unprotected.

sub Auto_Open ( )
ActiveSheet.unprotect Password:="mypassword"
Range("myrange").Select
Selection.Locked = True
ActiveSheet.protect Password:="mypassword"
end sub

have fun...