lock cell and/or message box after user enters data?

Morning,

I have a request log with data that needs to be protected once entered as it is 'signed off' so must not be able to be changed once signed.

Is anyone able to offer some coding that looks to check that once 'signed' the data in the cells of that row cannot be edited and/or a message box pops up advising unable to edit?

I have used various codes below to get parts of what I want but nothing to check against the signature cell?

Message Box:

Please Login or Register  to view this content.

Locking cells/sheet:

Please Login or Register  to view this content.

Any help would be great!

many thanks

p.s my signature cells are part of a name range "Signature", columns L-N, P-Q.

all other cells which need to be locked once signed, are in columns A-K, also named as a range if needed "Data"