You remove all info from your profile, change any options in your UserCP that you want modified, log out, clear your cookies and don't come back. If you have posts that you'd like to remove from your posting history, you can go through and edit them down to at least 5 characters (e.g. 'deleted' or five periods), but that will not affect posts in which other members have quoted the posts you'd like to delete. However, if this is your only account and first post, you obviously don't need to take that step.
In very extreme cases, some other steps can be taken, but if it's pretty much a case of not enjoying the site, or like your significant other not wanting you on the site, then you'll need to take the steps above and just walk away from the account.