One may not answer אמן to a ברכה until the ברכה is totally finished. Too often the חזן lengthens out the last word of the ,ברכה particularly on ראש חודש or יום טוב and the מתפללים answer before the חזן is done. Where is the source for not answering אמן to a ברכה until it is totally finished, and what can the חזן do to avoid this problem?
It's called an אמן חטופה and the Rambam (Hil. Brachot 1:14) already codifies it.
The source is from Brachot 47a. As the Kitzur Shulchan Aruch (6:10) defines it:
See Shulchan Aruch 124:8 that אמן חטופה seems to have 2 meanings: Either swallowing the Alef or answering too early.
The חזן can educate the congregation - for example, pausing before the last word - while the early birds answer Amen - and then singing the last word. After a while the congregation will realize.
As in "Hamevarech Eth Amo Yisrae-e-e-e-e-e-e-l" - pause while the early birds answer Amen - "BaShalo-o-o-o-o-o-o-m". Ame-e-e-e-en.