This would be an opportune time to educate said players.
If they are on your team, provide a little constructive commentary regarding their power's KB and affect on team dynamics. Positional use of their powers, blasting runners, etc. You are not telling them how to play, just giving them advice. They may be new players or maybe they don't normally play powersets that have KB components, hell they could just get a kick out of watching mobs go flying. I know I thought it was hilarious as all get out when I mez'd Scirocco and he clocked Ghost Widow during an STF even though it nearly caused a team wipe.
I can see one of four outcomes:
They may thank you for the tips and modify their plan-of-attack
They say nothing and modify their plan-of-attack
They say "Piss off, I'll play my character how I want" and continue blasting gathered mobs
They say nothing and and continue blasting gathered mobs
At which point the team lead can decide to keep said player on their team, finish out the mission and kick them from the team, or kick them from the team immediately and likley place them on their block list with a 'Not a team player' note.
Am I thrilled over KB? Not really, but I regularly team with friends who play Khelds or powersets that KB. I don't gripe at them about it so why would I gripe at someone that's not my friend. And I have to say that in my time of playing on Legacy and here on Homecoming, I've not met a player that intentionally griefs the team they are on with KB.