Linear optimal gains are computed for the subcritical two-dimensional separated boundary-layer flow past a bump. Very large optimal gain values are found, making it possible for small-amplitude noise to be strongly amplified and to destabilize the flow. The optimal forcing is located close the summit of the bump, while the optimal response is the largest in the shear layer. The largest amplification occurs at frequencies corresponding to eigenvalues which first become unstable at higher Reynolds number. Non-linear direct numerical simulations show that a low level of noise is indeed sufficient to trigger random flow unsteadiness, characterized here by large-scale vortex shedding. Next, a variational technique is used to compute efficiently the sensitivity of optimal gains to steady control (through source of momentum in the flow, or blowing/suction at the wall). The bump summit is identified as the most sensitive region for control with wall actuation. Based on these results, a simple open-loop control strategy is designed, with steady wall suction at the bump summit. Calculations on controlled base flows confirm that optimal gains can be drastically reduced at all frequencies. Forced direct numerical simulations show that such a control allows the flow to withstand a higher level of noise without becoming non-linearly unstable, thereby postponing bypass transition. Finally, in the supercritical regime, the same open-loop control is able to fully restabilize the flow, as evidenced by direct numerical simulations and supported by a linear stability analysis.