You have to just tell them. This notion of allowing folks in the name of friendship speak to you anyway suggests that you are keeping them even in an unhealthy state just to keep them around. That’s the same notion used in some romantic relationships just for the sake of not being alone. I know you want to preserve relationships and of course blowing up at everyone is frowned upon but real friends should be open enough to speak the truth in love. I think sometimes we forget that keeping real can go left sometimes and you must have a little finesse when you deal with other humans. People have things in their life and on their heart that not everyone needs the in your face approach or the let everything happen approach. There is a balance but you are the ONLY one who control that.
From what I gather you have allowed a build up go on way too long and you need to address the relationship as a whole. It sounds more to do with if that friendship should stick than it does with tone. As adults you aren’t going to necessarily get it right all the time, but friendships are a choice unlike family. If you choose this relationship in more than one way, then you need to find out what attracted you to it in the first place. Keep in mind that not everything can remain. So that means that there are times when relationships can run its course. You don’t sound as if you are just someone in a tiff with a friend but more you are recognizing the unhealthy part of the relationship. You have two choices, keep some control of how you allow folks to speak to you, or let it continue while you get frustrated and mad that you didn’t speak up. I would suggest you speak up when things are being said and if that doesn’t work, you may need to figure out the extent of the relationship and let it take its natural course. Also you have to own some responsibility. I have had friends who for various reasons have crossed lines, but it’s not the habitual line stepper that was the issue it was my issue for not making others respect me.