How to uninstall - django.db.migrations.exceptions.IrreversibleError

I am trying to uninstall `knox` by first running:

```
python manage.py migrate knox zero
```

but am coming across the following error:

```
django.db.migrations.exceptions.IrreversibleError: Operation <RunPython <function cleanup_tokens at 0xffff5ff11000>> in knox.0006_auto_20160818_0932 is not reversible
```

I'm not sure the best route forward here? 
Many thanks,