Blizzard loves gold farmers and real money sellers since bots are blizzard's largest sub base.
So, since 8.0, Blizzard started to restrict LFG APIs and did not even allow us to filter spamming in title/description/voicechat for LFG group.
The only possible way to block spammers is to blacklist their player names.
In Chinese Region, 99% of LFG groups are spammers. Although AddOn LookingForGroup can do a great job for filtering 90% of spamming by default, some spammers can still spam.
This AddOn will set a table for blacklisting them in LFG.
YOU MUST INSTALL LookingForGroup AddOn First.
Only usable in Chinese Region