Heat - welding torch, or MAPP Gas torch. Heat the bolt head to to hot. The heat in head will go down the shank some. Use big impact wrench and back it off. If that does not work, 1/8" cut-off disc in grinder and cut the blade off in two pieces so you can then remove the spindle assembly out through the deck and take to machine shop to get the bolt out.
Decent used spindles are $50 on eBay. So if the bolt is permanent, you can drop another in place and a new blade and you're good to go. If the shop can get the bolt out, new blade and good to go.
In any case, they use a compression washer to hold the blade under the bolt head. So use liberal amount of Never-Sieze in those bolts so they can be removed in the future ...
Worst case you must use cut-off wheel to take the bolt head off so you can get blade remnants and compression washer off spindle so it can be removed. Then heat the remaining bolt shaft stub hot and use pipe wrench to grab the stub and back it out ... If you have to go this way, I'd soak the bolt stub spindle end in a shallow can with diesel for a few days before I tried heating and backing out ...