How can I disable plugin upload and installation via WordPress Admin?
Should still be able to activate/deactivate plugins in admin.
There is a constant you can define in wp-config.php to do this.
It will also disable the theme edit, however.
<?php
// somewhere in wp-config.php
define('DISALLOW_FILE_MODS', true);
That will remove the plugin and theme file editor (which are a terrible idea anyway) and remove the ability to install plugins and themes from the admin area.