What you can do is make banish the spell, and, by modifying the gameplay contants, decrease the regen rate. There's two things wrong with this though,
1) the unit will still be ethreal, so it cannot attack and will take extra damage from spells and magic.
2) any spell that makes something ethreal, including the transform for the spirit walker, will make the unit have less regen, because it's ethreal.
Another idea would be to modify healing ward. What you will do is make the ability healing ward have a target, making it required to have to be cast on a unit. Next, reduce the aura for the ward to about 50 or something, so the aura only effects the unit it is cast on. Also, make the healing rate negative, so it won't heal the unit, but hurt it. This will also be level-able, sice you make a ward with a lvl 1 aura, lvl 2 aura, and so on.
Speaking of reducing regen rates though, I was wondering how to add a spell to a unit with triggers. The thing is, is that I have a spell which increases friendly life regen, and lowers enemy life regen. It's a hero spell, so it has levels. I've done everything, except find a way to add the spell. I need the trigger to be able to detect at what level the ability is, so that the proper degen aura can be added. I haven't found a trigger that does that so far. Any help would, well, help. Thnx.