by default I restrict avatar upload to the register members.(see my settings in screenshot),but some member are able to change avatar with my default forum avatar files.how can I stop that???
That setting disables the usergroup to upload avatars to your forum (which are located in your avatars folder), but it doesn't disable them to use externally hosted avatars.
You need a plugin if you want to completely disable a specific usergroup from using avatars.
There is a way to remove avatars completly just see AJS post
HERE.
But i guess to remove the option like you want then a plugin maybe needed.
Maybe Yaldaram has now got a plugin for this as per the thread above, maybe he will post here or either ask him or look at his site.