Micrometer-size graphene ribbons are generated by epitaxial growth on SiC substrates and contacted by electron beam lithography. The isolated graphene islands are patterned at nanometer scale by atomic force microscopy (AFM) under the application of an external polarization to the graphene layers. Contrary to previous reports, the patterning can be made at positive and negative polarizations and using significantly lower absolute voltages. The technique is used to tune the electrical resistance of the graphene ribbons. Combination of graphitization of SiC and AFM nanopatterning is, in consequence, a powerful approach for the fabrication of prototyped graphene-based nanoelectronic devices.