We have no bannister down one side of our stairs, wall on the other.
We have barricaded the bottom off almost completely; so there is a thick sheet of ply (about 60 cm high and the whole stairs wide) on the second step, screwed into the wall on one side and on the other side into a beam that extends down from the wall above.
We have to climb over the barrier or up hoik up the side of the stairs (straight onto the 3rd step) to get up.
Other people look at the barricade and think it's nuts, but we (age 4-42) are all fit enough to clambour around it easily enough. There's no worrying that someone left the gate open, which is a problem we have a lot at the top of the stairs. It's cheap and cheerful and strong, made from spare materials we already had, completely keeps the toddler off our stairs, and won't leave too many marks on walls when we remove it completely.