Hmm. If you don't have them, I would get some isolation pads (Mopads etc.) for your monitors to decouple them from your desk or floor, if you don't have them already. Sometimes a desk or floor can act as a resonator and pass vibrations on to a neighbor. Other than that though, like the others said, there aren't much in terms of sound treatment that will help with that particular problem.
You should write him a note to "please use earplugs after 7 pm."

Seriously though, I try very hard not to be an annoying neighbor, but there needs to be reasonble compromise here. My advice would be to measure how loud you play your music with an spl meter, and then tell your neighbor that you ARE willing to change your habits for HIS benefit. Something like the following
1) You purchased isolation pads (be sure to convey the cost) to help with the situation
2) You will only listen to louder untill 7pm
3) After 7pm, you will turn it down to 72 db (or whatever you choose as tolerable), a reasonably quiet volume
4) After 8:30pm (or even 9), you will don headphones. Tell him you just bought some for him.
Inform your landlord of all your sacrifice and comprimising as well. Look at all the the stuff you're doing for your neighbor! What a great guy you are!