Lock cells when they reach a max value?

[Ruiao6]

Hello,
First time poster. I'm having troubles finding a solution to my problem.
I'll have column A with description of the values and column B with values. The thing is, I need to lock any more adding of values when that reaches , let's say 8k.
How do I do that without a macro? Can I enter different descriptions (and therefore values ) interspersed and keep the value counting?

Thank you very much for your help!!

 

[mart37]

Maybe you can use Validation.