You could try taping up the holes on the inner sides of the earpads, but not all of them. Only the "bottom" half or third or so. It is the bottom part of your outer ears that pick up bass, is why.
I've seen head-fiers tape up all of the holes completely, if you really want to try that much. I haven't tried it myself, but I figure it is probably too much and will mar the mids and treble quality and clarity, hence I recommend leaving some open.
How the ear picks up sound even continues to hold true with on-ear headphones, in my experience, even though parts of the outer ears end up getting practically omitted in the "audio chain" going towards our eardrums.