There are a couple ways you can do this. Either Protect the Worksheet or use
Data Validation.
To Protect the worksheet, select each of your cells that you DO want data
entered into. Format the cells and choose "Unlock" from the Protection tab.
Then, select Tools --> Protection --> Protect Sheet. You may enter a
password or not, depending how secure you want the worksheet to be. Now only
the cells that you unlocked earlier will allow data to be entered.
To use Data Validation, select the cells that you DO NOT want data entered
into. Select Validation from the Data Menu. Under Allow, change "Any Value"
to "Text Length" and set to "Equal to" and 0. Now, any typing will be
prevented in those cells.
HTH,
Elkar