If they're intentionally picking something weird with the sole intent of losing and feeding with it as vengeance for their pick getting banned, that can be considered griefing and so just report them.
However, if it's a normal and they're just picking something weird and off-meta for the fun of it, then who cares? If normals are deathly serious to you, then just dodge.
Also, if you're just being a jerk and intentionally banning the person's pick because you either don't like that champion or you think they're going to suck at it, then I'd honestly consider THAT to be griefing.
Either way, just deal with it, report at the end, or dodge if you absolutely must, and remember to know the difference between griefing and just trying a weird build for fun.