We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z ≃ 7 "z–band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we report on two newly discovered galaxies at redshift ∼ 6.7 in the NTT deep field: both galaxies show a Lyα emission line with rest-frame EWs of the order 15-2 A and luminosities of 2-4×10 42 ergs −1 . We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Lyα EWs (between 20 and 5 A). We conclude that the trend for the fraction of Lyα emission in LBGs that is constantly increasing from z∼3 to z∼6 is most probably reversed from z ∼ 6 to z∼7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order ∆χ HI ∼ 0.6 in a time ∆z ∼ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.