Feedback problem

Recently I helped run an event that used Asterisk and Cisco VOIP phones to allow the staff to communicate from anywhere in the building. This was spectacularly successful and I have to give kudos to the Asterisk team for building such an excellent program.

However, one of the requirements of the system was that users could call a computer connected to the public address system and make announcements from their desks. This worked flawlessly until one of the users accidentally enabled speakerphone - since they were within earshot of the public address speakers it rapidly caused a feedback loop to build up. Luckily it was stopped before any of the equipment got damaged, but it would be very preferable for future events to find a way to stop this.

I’ve googled around a bit and can’t seem to find any useful leads. Is there a setting in Asterisk which can help suppress feedback? Can someone point me to any relevant resources?

Thanks