I would introduce it thusly:
Achtung, grammar schweinhunds! For too long zis forum has been plagued by inconsiderate and incorrect use of ze vords "your" and "its", sloppy spelling und vith a callous disregard for our friend ze apostrophe. Vell, zat is about to change!
The Grammar Nazis are in charge now, und ve vill be vatching you very carefully indeed... clean up your act or you may expect a visit from us in ze dead of night!
Zat is all. You vill not see nor hear us, but rest assured: ve are ALVAYS vatching! (We might want to can the German "v for w" idea, it's a bit mad but fun still...)
Also: vin fantastic prizes by turning in anyvun you know (or suspect, ve vill accept suspicion, substantiated or not) of using bad grammar or spelling.
Remember, it is your duty as a responsible citizen! And you know, ZEY voudl turn YOU in in a SECOND.... ;)
Oxfordis in Extremis!