To make sure no space flight, just make the planet a little heavier.
Based on chemical rockets, the cost to LEO is pretty terrible. If you increased the planet mass by a relatively small percentage, creating a chemical 3 stage rocket to deliver a payroll becomes prohibitive or even just impossible. 25% more mass for the earth would do it.
Many other lift designs are possible, but we use chemical rockets for a very good reason, the other designs have even less thrust, or have other negative feature (pulsed nuclear bomb a.k.a. Orion comes to mind).