How can I delete PS3 trophies?

No, it is not possible to delete trophies as soon as they have been uploaded to the Playstation Network (e.g. If you are always online, it happens as you get them.)

However, if you are not connected to the internet, there is one DRASTIC option, that also WILL DELETE ALL SAVEGAMES and all non-synched trophies, and that is to format your internal harddrive.

But, other than that, there is no way to delete a trophy one it hit PSN

I've seen this as well for demos on PS3, specifically PSN games. If you don't have it synced yet then yeah, they can be deleted. If you've already synced by either viewing your profile or your trophies then I don't think you can delete them.

I wish you could though as I've seen the same thing for games I don't plan on buying.