Text fields (varchar and nvarchar) are automatically available for Rich Text editing if the field is at least 100 characters in width.
To enable or disable this feature, in QAComplete:
Go to > Setup > System Configuration > System Options.
In the System Options (Affects all Team Members) section, use the check boxes in Rich Text Editor to enable or disable Rich Text editing for the related module.
For custom fields, you can disable rich text editing, no matter the size of the field, by using an underscore in the field name (for example, My_Field).