users can input data, but not delete it
No idea if you can do that. Have you considered using a form for the data input?
The method I mentioned before actually works. Almost.
Basically, all cells are formatted so they aren't locked (right click on them, formatting, un-check the box. Something like that). Then you lock the whole sheet (but the cells aren't locked yet, see), and what the macro I quoted before does is when ever a cell isn't
empty it will check this box automatically for you, meaning to have to unlock the whole sheet to edit data. (It's a macro that's always on, not one you activate as and when you need it)
But this is no good without a password to stop people unlocking the sheet and editing the data. But if I just add a password to the sheet it needs it halfway through the macro, so only the person who knows the password can enter data, which is against the point. So I need to enter the password in the code. Somewhere. I'm not sure where.
What I might do I record a macro of me running the macro and entering the password and finding what code this makes, but I can't do this until tomorrow.
God damn, I hate only knowing enough to know I don't know what I'm doing.