Feature creep is the uncontrolled addition of features in a product beyond what was originally agreed (and/or needed). It causes slippage, overtime and dilutes the original vision.
Usually, this happens when team members take on additional work directly from stakeholders, bypassing the Producer (responsible for the content) and the Project Manager (responsible for the delivery).
You can't stop stakeholders from talking to the team, but you can prevent work from being assigned that way: to avoid feature-creep you need to control the supply chain of work.
1) The Project Manager can only add items to the backlog if they have been approved by the Producer.
This ensures the Producer is aware of the change and can veto the work (or find an alternative) if it doesn't fit their vision. The Producer remains in control of the content.
2) Only the Project Manager can add items to the backlog
This ensures the Project Manager is aware of the additional work being assigned and can veto the work (or work with the Producer to find an alternative) if it jeopardises the schedule. The Project Manager remains in control of the delivery.
3) The team can only work on items from the backlog
Each team member is accountable for delivering the items they added to the backlog during sprint planning - and this rule still applies if they decide to take on additional, unplanned, unapproved work after the sprint planning. They may have to work late, weekends or both but the content of the sprint must be delivered. If that is causing too much stress and overtime, they
So when a change request is put to the team directly, regardless of who it comes from (client, stakeholder, even the CEO of the company), they need to report it to the Producer or the Project Manager so they can discuss and assess the work together.
The team cannot say (or even suggest) to the stakeholder that the work will be done: "I'll do it this afternoon" or even "Sure!" are not acceptable answers because they just commited to doing work neither the Producer nor the Project Manager have approved. "I'll tell the Producer and the Project Manager and they'll get back to you" is the only acceptable answer.
Let your team know the three rules, let them know (and practice) giving the answer, and most importatly, let them know that you will back them up if the stakeholder complains that they cannot assign work directly to the team. You're all in this together, protecting the team and the work from external influences.