There are some really good elements to this answer, but also a few things which are prone to misinterpretation or wrong implications. (1)
"and then ask them to sit down, and behave seriously" Asking someone to behave seriously, when phrased like that, can come across as arrogant and/or patronizing. (2)
"Tell them about multiple detailed accounts" I would suggest for OP to address problems they faced rather than point out bad behavior ("being unreliable") - the latter can come across as combative or accusatory. —
Flater 18 secs ago