It is not possible to remove an avatar from displaying but it is possible to change it. There are a couple of options I might suggest.

The first is to use a transparent image, making it look like you have no avatar. There is a transparent image hosted by yuku that you can copy, upload and apply. It is only 5px square, so not so easy to right-click and save. >Image< (between the arrows)

http://static.yuku.com/common/bypass/images/transparent1.gif

My second suggestion is to use the original balloon-head. It can be found here:

http://static.yuku.com/common/bypass/images/missing_avatar.gif