Then add the option, but make it default to soft caps. Or bake the default into the tiers. If I sub with a $20 spending limit, I will probably be ruined by a $2000 bill. On the other hand, if a company subs with a $1000 limit, it can probably cover the occasional $10000.

At the very least, there should be an optional hard limit that is obviously indicated in the UI. When you're signing up where you set your "usage cap" warning, next to it should be an optional hard cap with big bold red letters "THIS WILL CUT YOU OFF THE MOMENT YOU GO ONE CENT OVER". So I can set e.g. a warning at $20 and a hard cap of $100.