I'm probably over-thinking this one... but how do I have one cell that is locked for access when the value on another cell is greater than 1?

Example below explains my requirements better.

Column
A B C D
Row 1Level Cost Time Resource
2 1 £5 1:00 0.5
3 1.1
4 1 £7.50 0:30 1.0

I need to lock the cells in Columns B C & D for any sub-level (i.e B3, C3, and D3) in order that the user cannot input any value.

Anyone able to offer me any guidance?

Thanks in advance.