It's getting absolutely ridiculous. And SL is a small world :( We'll be wrecked if this platform grows anymore.
Couple mitigating strategies, since I know you know this is a problem, but the problem persists:
IF
* Account less than X days old
* Account sends messages to more than G large public groups of size S in M minutes
* Account asks for #L, example 100L or 150 "linden dollars" or "100 lindens"
THEN
Account is banned for 2 days, increment per event.
You can, I expect, do a bit of data crunching to determine the best values for X, G, S, M and a pattern for #.
This would solve quite a lot of it, making it just vanish. For patterns that don't fit this (eg, established accounts getting hacked and used to spam for Lindens), you want the actual AR process so that someone doesn't lose their Second Life just bc it got hacked once.
For bonus points, rather than a pattern of number + L you could do a bit of actual nlp, testing "is this message asking for money." But even just a broad pattern search, in combination with the previous factors, would be spectacular.