The suppression of the amplitude of spectral resonances related to exciton states with large wave vectors is observed in the reflectance spectra of heterostructures with wide quantum wells in a transverse magnetic field (Voigt geometry). This effect becomes stronger with increasing exciton wave vector. This phenomenon is caused by the weakening of the exciton-light coupling and the ionization of excitons by the effective electric field arising owing to the magneto-Stark effect. An accurate microscopic theory that describes the exciton-light coupling of states with large wave vectors in a transverse magnetic field is developed. The reflectance spectra calculated within this theory show good agreement with the experimental data.