Anyone's situation can change, or you may find that a particular campaign simply isn't a good fit for you. Regardless of the reason, sometimes you just have to walk away. If you no longer wish to participate in another user's campaign, you may leave at any time via your profile page's Campaigns listing.
To leave a campaign:
Navigate to your profile page.
Click the "Edit Your Profile" link.
Locate the campaign in your Campaigns listing.
Click the "Leave Campaign" button.
Confirm that you wish to "Leave Campaign" in the dialog window.
Upon confirmation that you wish to leave, you will be removed from the player roster. The campaign will be taken off of your profile page, and ownership of any characters or items that you created for that campaign will be transferred to the campaign's GM.
Note: Should you ever wish to return, you will once again need to petition the GM for an invite to the campaign (see Joining a Campaign).