For the stuck on ones it's best to heat them up with a hair drier until the glue goes soft and then just carefully pry them off. For clipped on ones, remove the locking clips from the otherside of the panel, then gently tap the legs the clips were on to push the badge off.
For the stuck on ones on the tail, i used a constant feed of very warm water
& an old e-top up card to gently ease them off mate
i used the plastic card as it doesn't scratch the paint at all
the front 'P' or 'Nissan' just pops out as Ben sais