I have a portal on a layout.  

 

I want to conditionally disable the ability to enter information in one of the fields based on a value in a related table on a row by row basis.

 

Is this possible?  Suggestions?

 

Thanks in advance.

You could disable entry to the field and turn the field into a button running a script. The script would either go to the field or do nothing, depending upon the condition.

 

Note that this is a purely cosmetic device, working at the layout level only. If the reason behind the restriction is security or data integrity, you should be using the privilege set to enforce it at the data level.