Take uke where there is no support, apply downward pressure and sit down. If you push against the ear, you leave uke on their feet and in control of their own body. They let you do ikkyo to them.
Also realise that if uke wants to escape, they will, why do you want to do waza on somebody that are retreating.
Fair point, even if I'm sure one could come up with some scenarios where one would want to.
I have been bugged by the technical intricacies of of trying to stop uke from rolling even if I can see the combat-strategic advantage of letting uke do the roll if that's what he's hell bent on. There are many possibilities down the roead then, probably simpler and safer than trying to stop the roll.
Go with the flow I suppose.
All the best