How can I cut a bolt off flush with concrete in a corner?
Since it appears to be in dirt, dig it out would appear to be an option. If there's actually concrete under the dirt, chisel it out is still an option.

I'm unclear as to how that's not accessible to a hacksaw - looks plenty accessible to me. Other than a hacksaw, a sawzall (reciprocating saw) or angle grinder with cutoff disk are typical powertool approaches. An oxy-acetylene cutting torch would be another way.