Can sunscreen prevent tanning if used correctly?

Yes, if you apply a generous amount, reapply every two hours, and cover all exposed skin, sunscreen will prevent tanning to a large degree. No sunscreen blocks 100% of UV rays, but SPF 30 filters about 97% and SPF 50 about 98%. The leftover 2–3% can trigger some pigment if you’re out all day, but most tanning from sunscreen is due to user error — not applying enough or reapplying too late.