"I would like a pizza without olives and onions."
Translation:Je voudrais une pizza sans olives et sans oignons.
"...without olives OR onions". Otherwise, the subject is asking for a pizza without both, rather than emphasizing that neither is acceptable. Though technically "correct", it is quite awkward in its linguistic logic, and I doubt any native speaker would say it. SnarfSnarf123 answer below is good, however in this case, with only two things listed, "une pizza sans olives ni oignons" would be best.