I can't un-employ a villager after trading with it. This is the same for all type of jobs that they can have (ex: farmer, librarian, etc.)
Related issues
Comments

Please also note that general gameplay issues with the Java Edition should be reported in the MC Project.
Thank you for your report!
We're tracking this issue in MC-153617, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki