It is not possible to disable Accept merge request on success -feature.

Reproduction steps
1. Create an new freestyle project
2. Select git repo and "Build when a change is pushed to GitLab"
3. Unselect all checkboxes under "Build when a change is pushed to GitLab"
4. Save
5. Go again to configuration
6. Accept merge requests on success is checked.

There seems to be no way to disable the functionality, the checkbox always becomes checked. I also tested that merge requests actually get accepted when build succeeds.

Unconditional auto accepting merge requests is totally unsuitable for our workflow.