I have read that Mr Surfacer can be cleaned up with alcohol, but it doesn't seem to work for me.
I'd second the "Mr Color thinner" because it's a styrene-friendly lacquer thinner (won't melt your model). Make sure you get "Mr Color" (lacquer) thinner, not "Mr Hobby" (acrylic) thinner.
On the other hand, Tamiya's version (Liquid Surface Primer) cleans up well with alcohol. (Well, ethyl alcohol anyway)