Bellflower has running rhizomes. They can go really deep... so you keep thinking you dug them all up, but there are always more. There's no way you can dig them up and actually get rid of them. I have Dark Towers penstemon which is similar to Mystica - I love it. I usually cut the stalks once it is done blooming, so I haven't had any issue with it self-sowing. I wouldn't mind if it did!
Another campanula you could try would be nettle-leaved bellflower 'Bernice'. That again has bell-shaped leaves but gives somewhat of a vertical effect. I have posted some photos of it on this site. Still, none of these have the deep color of May Night, and the bellflowers would not either. They are usually a lighter shade..
There might be newer cultivars of the salvia that are taller than May Night. Madeline or Caradonna maybe? I haven't looked lately.
