Locking cells so formulas are never removed

I have built a multi-page spreadsheet with many formulas which utilize data from other cells throughout the entire spreadsheet. Now that I have completed the spreadsheet, I would love to be able to lock the cells with formulas, while still being able to plug (more like copy and paste) data into cells which are referenced. Is there a way to do this?

Additionally, is there a way to lock a cell formula where you can "delete" or "clear" the value in the cell without erasing the formula in the cell? For instance, here is an example:

A1 contains a plugged in value of "10".
A2 has a formula which states: =(A1*-1) which returns the value "-10".

I would like to be able to highlight the entire column and erase the numbers plugged in manually, and in return the value displayed by the formulas, however, I do not want to erase the formula in the cells. Is this possible???

Thanks ahead of time and my compliments to all the quick and helpful responses I have received on this website!