Need to be able to enter data in a blank cell but not allow anyone to delete the

I [font=Segoe UI, Helvetica Neue, Helvetica, Roboto, Oxygen, Ubuntu, Cantarell, Fira Sans, Droid Sans, sans-serif] am fairly new to Excel. I need to create a sheet where the rows are the days and dates of any given month.  I need all cells to allow anyone to enter data into a blank cell on any given row but not be able to delete the data once entered. Ideally the only function I would like is to allow the user to be able to perform if desired is a [/font][s]strike through [/s]once data is entered.

Please help and thank you in advance. 

Paul