I have a spreadsheet where people are going to type in numbers, and
the final cell is a total, which of course I don't want people to
accidentally type over, as it will lose the formula behind it.

So...... I put some "data validation" behind that cell, to say only
allow entries of text length = 50 - a kind of "mock" condition.
Nobody'e ever gonna type in 50 chars. are they?! And sure enough,
typing in (eg) 854221 brings up the error box to stop them, just like
(eg) HHJSYT brings up the error box to stop them. I even unchecked the
ignore blank" box, so that entering (eg) " " does the same.

So much for numbers, letters and spaces. HOWEVER, if someone goes into
the cell and presses delete, delete, delete (i.e. making a "null"
character), it allows this! Help! I can't think of any way round this.
Can anybody suggest one?

Thanks.