This must be an easy question for those who know the answer.
May be, but you should place your question in appropriate community or it may remain overlooked and unanswered. I moved it already.
Can I disable certain lines, extensions with the phone itself without accessing the web interface?
Yes and no (rather no in your case). You can modify configuration by either WWW UI or by provisioning. It may be possible to arrange the environment so service code dialed from phone will provoke the provisioning with required modification.
In the case you need to switch between just two configurations only you may (mis)use Mobility feature for the purpose - but even in such case the provisioning is required.
