There is currently a "+" button to easily add oneself to the Nosy list 
when logged in in the web UI (at least that I see on the bugs.python.org 
roundup tracker).  Please make this turn into a "-" button to remove 
oneself from the Nosy list when the user is already present.

This is specific to our bugs.python.org instance and it's not in the
default Roundup (I would be willing to contribute this and other UI
enhancements if people are interested).
The reason why there's no [-] button comes from the wider goal of
keeping the UI as clear as possible.  Adding oneself to the Nosy is a
very common operation and it deserves a button, whereas the opposite is
fairly uncommon as far as I can tell.
See also http://psf.upfronthosting.co.za/roundup/meta/issue266
(especially msg1677).

Suggest closing this here (unless you want to turn the issue in a rfe to
add the [+] button).

yes I'd turn this into an RFE.  I made this request as the bug tracker 
we use internally at work gained the + and - feature for its equivalent 
of a nosy list and there was much rejoicing among all users.

As for supporting -, I'm suggesting that the button change based on the 
users presence in the list or not.  It isn't difficult to support both 
and people do eventually decide they don't want to hear from some bugs.  
but agreed, + is more often used than -.

(granted you could also consider this an RFE to upstream some of the 
changes we've made in the bugs.python.org roundup tree.  less changes 
for us to maintain and others benefit.)

> As for supporting -, I'm suggesting that the button change based on the
> users presence in the list or not.  It isn't difficult to support both 
> and people do eventually decide they don't want to hear from some bugs.

Having the [+] switching to [-] is trivial to implement and it's
reasonable, so for the regular Roundup it might be OK to implement it
like this.
With bugs.python.org we are really trying to avoid adding other elements
to an interface that is already too complex -- unless they are really
necessary.

(Another thing I was considering for b.p.o was to extend the
autocomplete to show a dropdown with checkboxes when the user clicks on
the nosy, so that it's easy to add/remove any user (and not just oneself).)

I would also be interested in seeing this feature added to roundup
proper.  A single button that is + when the user is not in the list, and
- when the user is in the list sounds right.