Once a creature you control has been blocked, activating The Destined Thief's second ability targeting that creature won't cause it to become unblocked.

If a creature has more than one party creature type, and there are multiple ways to count that creature that could result in a different number of creatures in your party, the highest such number is used. For example, if you control a Cleric and a Cleric Wizard, the number of creatures in your party is two. You can't choose to have it be just one by counting the Cleric Wizard first as a Cleric.

To determine "the number of creatures in your party," check whether you control a Cleric, whether you control a Rogue, whether you control a Warrior, and whether you control a Wizard. The number is the total number of those checks to which you answered yes. Each creature you control can be counted for only one of those checks.

Some cards refer to you having a "full party." This is true if the number of creatures in your party is four.

If you have a full party, you won't discard at all when The Destined Thief's last ability resolves.