scrape the big stuff off, then work down with some sand paper, or a metal kitchen scourer. You can also put glad rap on the guards to before doing some work that way you just pull the glad rap off and the tar comes with it
Clean the big bits off with your hands/fingernails, then go to repco and buy maybe 3-4 cans of pressure pack degreaser depending on how many pairs of tyres you've done (1-2 sets= 2 cans 3-4 sets=approx 5) I know you use more degreaser exponentially because of the potential incease of surface area covered in rubber. spray on and rub back with car sponge/old rag/ hand, it usually melts off. Don't know how it works but i've done it a few times now.