unable to reply to messages

How may I "clear space" in order to send messages? I recieve a message from the site saying Ive met a quota, and I'll be able to send messages when I can make space

Assuming you've read your messages:

Click "Private Messages" in upper right hand corner
Click the "Go" button next to your Inbox
Check all the boxes you wish to delete
Use the dropdown menu to select "Selected Messages: Delete"
click the "Go" box

Hope this helps