Disabling / enabling values
Something that is good to know is that when disabling a particular value, that value can still be the current value for existing issues. You can not disable these values selected in the past. Disabling a value means you cannot pick that value for new issues.
When editing existing issues with the disabled value, you are forced to choose another value before you are allowed to save.
How to disable / enable values
Navigate to the project
Go to Project settings
Go to Custom field values
Choose Edit behind the custom field
Turn the checkbox in the column Disabled on/off.

When you choose to disable the default value, the default value will be reset