Are there any rules for what you can put in your signature?

[Mabsark]

Could I change my signature to say something like "WARNING: X service is a ponzi" or would that be against some site rules I'm unaware of?

[hilariousandco]

I can't see why that would be against any forum rules. It may be against some signature campaign rules though as some don't allow anything else there so I'd check (assuming it's a paid sig).

[DiamondCardz]

Your signature can be whatever you like as long as it isn't advertising something illegal in your country, I'd assume.