After a bit of searching around, I have found that it is possible to remove them completely from your Google Talk list. Because all the answers I could find were incomplete and/or vague replies to forum questions, I'm writing the procedure down here for posterity. Do as follows:
- Log into your Google+ account.
- Sign into the builtin chat client on the right-hand side panel.
- Open the drop-down menu next to your name within the chat client and select Privacy settings.
- In the Choose who can chat with you option, select Custom.
- Unselect all the circle names in the checkboxes that appear.
- Click save.
- Witness all those useless contacts vanish from your IM clients! (Both native and web based.)