Why not add a one-click 'remove spam page and all contents and remove and permablock the creator' button/script? (assuming this is possible and relativity easy)

It's not built-in functionality, and so not easy.

Encouraging spammers to mess with actual pages people want to read rather than advertising their presence with obvious spam as they currently do doesn't seem good.

It may break their bots/scripts when they can't create pages as they obviously default to doing; and when spammers edit pages, they tend to not replace the contents but add to them. Since the wiki isn't heavily trafficked, it's a tradeoff I'm willing to make.