Search for the group of contacts you want to from from a particular category or categories by using the Quick Search or Advanced Search.
By doing this, you can return all the contacts you want to remove from a particular category.
Remember, to perform a function on a contact/or group of contacts, you must first "search" for the contact(s). Then, once you have retrieved the group of contacts to the contacts form, you can perform the action on that group of contacts.
Now, simply click on the "Remove Contacts From Category" button.
A new form will pop-up. Just select the category you want to remove contacts from (Using the arrow keys back and forth) and then click Remove.
This will remove ALL the current contacts returned by your search into the contacts from the category you selected.