BuddyPress.org

This isn’t hard at all.

Apply a filter to:

bp_gravatar_url

Return a url to a default image.

[EDIT]

Just noticed that bp_gravatar_url only changes the domain… the params are still added at the end of the URL.

Might try applying a filter to:

bp_core_fetch_avatar

Write a function to see if “gravatar.com” exists in the image URL, if so, return a default image.