We've run into the same problem. What's made this a particularly hard problem for us to address is that the intentions of involved parties are usually aligned with the best interests of the business in the short term despite being misaligned (and detrimental) over the medium and long term. In discussion it's clear that both parties are doing what they believe is right for the company, but their level and responsibilities might inadvertently encourage them to optimize for the short term. The ensuing conversation is usually a tight rope walk between trying to get both parties to understand short term vs long term outcomes and consequences while encouraging them to continue to execute on what they believe is right for the company.

What's worked for us is to not restrict communication, but to hold our engineers accountable to what they say they'll get done and when. If something slips and it's not for a technical or personal reason (i.e. some risk materializing or needing to take unplanned time off), but rather because someone floated in and asked for something that wasn't scoped in, that comes up in a review and reflects poorly on the engineer and the asking party. On the other hand, if the engineer is able to get all of their scoped work done and also get the unplanned work done, that reflects well on the engineer and potentially the asking party. In our experience, approaching these situations in this way also has the benefit of preserving and encouraging individual autonomy.