Prevent deleting data in cell WITHOUT protecting the sheet

[x65140]

EXCEL 2010: Cell A1 has 10, Cell B1 has 15 , Cell C1 has =sum(a1+b1)

The user highlights all cells and hits delete (which clears the data and formula)
(users do this even after you tell them not to)

Is there a way to keep C1 with the formula WITHOUT protecting the cell/sheet ?

I've looked a conditional formatting and data validation, but can't seem to get what I need...