Im guessing that it's freezing up do to the large amount of messages trying to be deleted at the same time.
First try going into the messages and long pressing on each message individually. When you do that, you will get a menu that pops up. And one of the options will be to delete message.
The other way to delete is to go into messaging, and long press on the conversation. Which will bring up a menu and in the menu gives you the option to delete.
If you use the first option to delete some messages first than once you have gotten rid of a portion of them that way, you should be able to come back and finish up the deleting of them by conversation next.
Let us know what happens.