It should be a simple thing: deleting email addresses and info for contacts you no longer want to see autopopulate in your Gmail account. While Google never added a contact listing for that cloaked Craigslist email address messaged once about concert tickets, the info still exists in your contacts, ready for you to accidentally mark as a message recipient. Time to clean the little black book part of your Google account.
There are a few ways to access a Google account's contacts. First, when signed into Gmail, click on the drop-down menu labeled Gmail in the upper left corner, under the Google logo. Select Contacts, which will then appear grouped into categories like Starred, Favorites, and within Google+ circles. The category Other Contacts is where all those random addresses will appear. Select any you'd like to delete, then click the More drop-down menu and press Delete Contact. Et voilà.
Alternatively, access contacts outside of Gmail with Google's Contact Manager. That'll take you straight to the same list of contacts found using the above Gmail steps.
If you have a change of heart about any of those deleted contacts, Google will let you recover them for 30 days, but after that deadline, they're gone for good.