Fleas do not feed on reptiles, they are strictly mammal (and sometimes bird) parasites. If your reptile has little black "bugs" in her cage, they may be mites. In which case, I would give an entirely different answer.
If they are just fleas and are only using the lizard's cage as a breeding ground, clean the cage thoroughly soap and hot water. All ornaments in the cage should be either thrown out, soaked in bleach and washed off completely, or sent through the dish washer (my favorite choice). All bedding should be thrown out. If the bedding you use happens to be Repti-carpet, put that in the dish washer as well.
A couple of other people here have suggested ceder chips and pest strips. Please REFRAIN from using these items as both have been proven to kill reptiles, who are much less tolerant to chemical smells than us mammals.