Restrict autoposting to centralized account by user group

Hi

I'm trying to set up twitter integration for my school blog and have configured a centralized account which all works great. What I want to do now is restrict this feature by user groups so that some users don't get the "Share" option whilst creating or updated a post.

I've tried setting the "Update Twitter" option within the ACL tab to No but this doesn't seem to have made any difference.

Thanks