Use the direct link to ‘close’ your account, which unfortunately means some data will remain publicly visible, and other will linger around in their database. This remains somewhat unclear. Judge for yourself:
We delete all of your posts, profile data, as soon as humanly possible. Your comments will hang around, but be associated with your Diaspora ID. You will get signed out and locked out of your account. This will lock your username if you decided to sign back up. Currently, there is no turning back.
What can you do: (1) anonymise as much as possible (see below), (2) go to your profile, settings, scroll down for the closure link to close your account and (3) if you fear the world enough, contact them personally requesting deletion of your data. For example, (3a) file an issue at https://github.com/diaspora/diaspora/issues or (3b) send feedback here: https://groups.google.com/forum/?fromgroups#!forum/diaspora-discuss
Also on the Diaspora development page at Github you can read things are not quite as they should hopefully be: https://github.com/diaspora/diaspora/issues/1823