What should I do with the mother-in-law's staircase?

[Lopex]

Have you tried using leverage on the bolts?

 

[ricebridge]

I would have used a reciprocating saw to cut the bolts; with an angle grinder, there's a higher risk of accidentally setting the house on fire

Not if you are careful. Some well-placed shielding and tactical use of a water hose solves it.

 

[Keld]

Not if you're careful. A little well-placed shielding and tactical use of a water hose solves it.

Pry off the boards with a crowbar, then cut with bolt cutters! that way it won't catch fire

 

[snoddas2]

Penetrating oil doesn't seem to have made much difference on the bolts, so I guess it'll be the angle grinder...

If you don't have the option/want to use an angle grinder or reciprocating saw, a regular hacksaw and some determination should do the job.

 

[quick]

A chisel and a decent hand sledgehammer can usually fix bolts quite quickly and easily. If you don't have bolt cutters, angle grinder, or similar tools available.

 

[osiris]

Yeah, but it's 100 f-ing bolts if all the planks are to be replaced. I'll probably be a bit selective...

 

[BosseHansson]

Isn't it just 80 of them? It can't take especially long to cut them off. More work to drill the new planks...