Use a dedicated tar remover for tar spots like mentioned - The most effective, and also cost effective ones are Autosmart Tardis or Powermaxed Tar Off. Just be aware that they will bleach black plastic and rubber. The way to get around this is to spray finely on the car and use a microfibre cloth to spread it. Leave to dwell for about 2-3 minutes and you should see the tar starting to break down.
Some tar spots are notoriously stubborn however, and will need a clay bar. Whilst the argument of "marring" is correct, if you're decontaminating a car, I would assume you're going to polish afterwards anyway in which case it doesn't really matter and despite what anyone says, clay barring is the most effective tar removal method
You may also notice brown rusty looking spots for which you need a fallout remover.
Worth remembering not to do any of this in bright sunlight as tar/fallout remover will dry on the surface, make it dull and need to be machine polished to remove.