When a record is inactive, changes to most fields in record are locked. Still, there are some actions which are out of box allowed like adding activities, notes, etc.
I often have had this scenario.
Here is a simple workflow solution to solve this problem. I have taken example of Contact record and Notes.
Step 1: Create a workflow on Create of Notes (It should be synchronous):
Step 2: Put a check condition to check parent regarding status:
Step 3: Stop the workflow with cancelled, if this condition is met true:
Set Error Message in Properties:
Now let’s see this in action:
Hope it helps!
MSDYNAMICSBLOG BY DEEPESH 
This is excellent, Deepesh, can be useful for another entity as well….
Hi Deepesh,
I tried to reproduce the same workflow in my Contact records. But I am not getting any error while I am entering notes to the inactive contacts.
Make sure you choose synchronous workflow
Thanks Deepesh. I changed the workflow to synchronous. Now it works.