That is why I use a fermenter that is only 60% full. That allows the foam to resolve itself and not clog the outlet. Also I believe that forcing that back down into the ferment can cause off flavours. I let it float untill the fermentation stops and it just sinks.
I never realised that you used glass Carboys. I shudder to think what would happen if the foam compacted in the neck. All that glass?. I think you all must have a death wish.
In Australia the tendancy is to use 25 litre plastic fermenters. I use a 50 litre for the reason above. Brew amounts are 22 litre generally. 30 x 750ml bottles.