Like someone taking a glyph when there is a low HP/mana Soraka on the team...I mean, really? Okay, I give them the benefit of the doubt the first time and tell them how my heals are way more effective than the glyphs. I ofc judge that situation individually.
Not everyone knows the glyphs spawn every 40 secs, and actually keeps track of that timer...
Intentional feeding is easily spotted - no point in bothering with telling them why that's a bad idea. Let them have their fun, don't say anything to them except perhaps trading jokes with the enemy team to make the most of it, and report the feeder afterwards.
Flaming at such people just gives them what they want: attention.
Also, it may be just me, but I got a big thing with surrendering in ARAM. I am very much inclined to report a person for negative attitude if he plays a tank, goes 0/13/7, even if not on purpose and basically just gives up and starts spamming surrender votes.
He may not be having fun, and I can easily understand his frustration (we've all been in that situation at some point in that mode), but he made a commitment when queing up, and he stands responsible for doing his part on the team until the end screen.
As you said, AD Morgana is perfectly acceptable if they know what they're doing - if they don't, and just do it out of spite, aka "it's only ARAM", they should be reminded of the rules: put the team before yourself: "Enjoy Yourself, but not at Anyone Else's Expense".
Anyone go "it's only ARAM", I report them, regardless of what their offense is.
I played my ARAM match #5000 not too long ago, and so I have seen my fair share of different attitudes there.
As well as far too many people who just can't suck it up and play even if they get a..."a derp champion", I believe was the term. And so they just dodge instead. I prefer to go 2/15/5 with Evelynn instead of dodging if there is no other way. At least I get experience with her, both in playstyle and build, so as to perhaps not make the same mistakes again.
After all, I did make the decision of unlocking her in the first place...
I like to think this mentality could be beneficial for a lot of ARAM'ers.