How to remove 'By term' and 'View' tab being displayed at top of pages?
'By term' seems to be generated by one of the views in Organic groups: posts_by_term. You should be able to disable it at admin/build/views. Here's the issue https://drupal.org/node/1635082.

The View tab should not be displayed if it's the only tab, for example for authenticated users who don't have permission to edit the node.

Removing the View and Edit tabs, should you want that, is a bit complicated because you still might need them in some situations. Here's a pretty thorough conversation about that: https://stackoverflow.com/questions/3530756/how-to-hide-edit-view-tabs.