sprout, I am not aware of a way of going this, but the people at Seattle may know of a way. On the Social Strata help forum there is a post which indicates that it is possible to do what you want via your Profile settings, but I've had a look at my Profile settings and can't see such an option. It may be they were talking about your own avatar which you can delete, but that's not what you mean.

Thanks Loro.   Yes, I've had a look in my settings but as you say can't find anything either