Excitation functions have been measured for the fusion of the weakly bound nuclei 6 Li and 7 Li with 209 Bi. The complete-fusion cross sections are lower than those predicted by fusion models, being only 65% and 75% for 6 Li and 7 Li, respectively. Within the uncertainties, this suppression is independent of beam energy. Distinguishing complete fusion from incomplete fusion, both experimentally and in theoretical models, is essential to understand the fusion process of weakly bound nuclei. A simple classical trajectory model which makes this distinction is presented. Further developments of the concepts of this model could be used for realistic predictions for the fusion of unstable weakly bound nuclei.