add a security rule for W4101B.

Hi, I'm trying to add a security rule for W4101B. One role has only access to SRP8 (Modify), an other role is responsible of item creation without access to SPR8 modificatiion. The issue I have is that this second role is creating item by copy. Then I'd like to find a solution not to copy SRP8 value (put blank). Does anyone have any idea how to do it? Thanks