There are times when it is appropriate in any forum for a topic to be closed.
It is not so much that it is done but how it is done.
1) Give a warning or two
2) Try to bring it back on Topic
3) A suggestion here, pm to ask politely to either edit or close when things become offensive.
4) Explain that you are closing and give a valid and considered reason for doing so.
Just a few thoughts/opinions.
They are my own opinions, not a code, not a rule just my views.
Cheers