Stick an Iron Golem in the way.
Also, I suppose, walls. Or underground, where blowing up most of the scenery isn't profitable. Or mob templates rather than lots of individual mooks. Or big bad things, like the Tarrasque, that have a good chance of actually surviving the overkill damage--hell, if its carapace still doesn't negate the line entirely (because I'm not sure if that counts under Final Spark's 'ignore total defences' stuff). Or applying Spell Resistance, because I'm not even sure if that does apply, and if it does we get a fun binary chance of negating almost every single ability I have.
Or just stick an AMF anywhere in the entire storyline and strip me of basically everything I can do, except for fly because that doesn't appear to be magical for some reason. Stick in a dragon, or something good in melee.
And this is only if I'm entirely on my own. If I have a party, there's this thing called 'collateral damage' to keep an eye on, where blowing up other players is incredibly stupid, and Blazing Star is at its most functional when used to keep out of reach--stuck in reach, it's a reliance on AC to not get hit. This also makes it much less profitable in a fight in smaller environments, where PC's would get caught in a 60' sphere.