Cannot delete some ring groups

You may try this:

select d.id  from ombu_destinations as d
left join ombu_destinations_category odc on d.category_id = odc.id
where odc.module_id = (select module_id from ombu_modules where name = 'ring_group')
and d.module_id = (select module_id from ombu_modules where name = 'time_conditions')
and d.index = (select ring_group_id from ombu_ring_groups where extension = '100');

The script above will return you the destination IDs associated with the ring group 100, so, now that you have the ID, you only need to delete the item:

delete from ombu_destinations where id = ?

The ID will be the value or values returned by the first query.