this is my first post so dont laugh at me bu to do what you want man in the profile.php file find the code
<label><input type="checkbox" name="form[use_avatar]" value="1"<?php if ($user['use_avatar'] == '1') echo ' checked="checked"' ?> /><?php echo $lang_profile['Use avatar'] ?><br /></label>
and comment it out and then
in the database for users set the default for useravatar to be a 1 instead of a 0
the database change will set all ppl to use avatar on creation commenting out the code will hide the check mark for changing it