Find all the windows where those attributes are and delete them. Then in object design right hand click on each attrubyte and delete. It doesn't do the delete until you save though. If the attribute is referred to anywhere else (such as in a query), you will get an error when you try to save the object. So I'd do then one attribute at a time, save, delete, save and so on.
in addition to dmshimself's instructions you might want to update all your Incidents setting the value of your attribute, that you want to remove, to NULL. Otherwise deleting might be impossible. The only catch here is, that you have to do that in an SQL query in your MS SQL Management Studio.
If deletion of an attribute fails and you have got rid of all the places it is used, then I'd raise any other issues you find with support.