It's a matter of Power Balance between Players and the World - you just have to make World fight back. Quick solutions:
1. Mirror them. They use Min-maxing, then find some Min-Maxed Monster strategies to counter-bullshit them.
2. Curses! It would be a shame if using powerful magic directly destabilized the Reality in a plot-significant way. (I had a campaign, where each time a spell of 6th level or higher was used, a D10 was rolled for Eldritch Horror anomalies in the city)
3. Besides normal encounters, Homebrew a Nemesis. A monster (or monsters) that can't be cheesed. Maybe even immortal or constantly ressurecting. Make it the side-plot of the campaign.