If a contact record has parents' evening bookings made against them for the past, present or future, you cannot delete their record from the Others page.

Replication Steps:

Modules > Diary > Parents Evenings > New > Setup a Parents' Evening > Select the newly created Parents' Evening > Actions > Manage Bookings > Right click on an available session > Manual Allocation > Select a student and a contact > Save >  Go to Others > Set the type drop down to Contact > Search for the contact you added an appointment against > Select them and click actions > Delete > Click Delete on the confirmation pop up > Once the process has finished, search for that contact again > The contact remains on the system