By default, all cells are "locked". However, this doesn't have any effect
unless the worksheet is protected.
So, unlock the cells where you *do* want input and then protect the
worksheet with a password. If it is only to protect the calculations or
reference data from being inadvertently corrupted, you could have a blank
password to make it quicker if/when you want to unprotect the worksheet.
Format | Cells... | Protection tab| Lock/unlock cells
Tools | Protection | | Protect sheet... | input and confirm your password
Regards
Trevor