I'm not sure how well this will answer your question, but if you go to Format annd then Cells and look at the Protection tab then you will see a tick box for locking a cell or range of cells.
Now this only comes into play when Protection is enabled - click on Tools and then Protection. You can limit what cells users can click on (have a play and experiment) and you can set a password which means you cannot edit a locked cell.
HOWEVER as I understand it you want the cells unlocked until the user enters something and then for a cell to automatically lock once data is entered. I'm not so sure about this, but if you had a macro which is triggered by hitting the enter key or on any data entry event (and I am not sure myself how to deliver this), then that macro could perhaps lock the cell you had just entered data in?
I hope these random thoughts will at least point you in a productive direction and I should be very interested to hear what working solution you come up with.
Good luck,
Caspar
PS Incidentally, another idea I have just had would be to create a user data entry form into which the user can enter one item of data at a time. Now a macro behind this would be easy and it could write the data into a protected sheet in a column which the user couldn't edit, but could look at and print - would that be any good?