I had gotten a load of bad hay this year. Before I realized there was no way my horses would eat it I kept trying different bales figuring maybe a dog peed on the bale. This left me with a bunch of wasted hay in winter making getting rid of it difficult. I tossed it along a bank near my barn. Which begs the question will this mulch down on its own or should I do something to it. At the bottom of the bank is a stream so I don't know how safe using lime due to run off to the stream. Id say there is 10 bales of hay broken and spread out as best I could.
Yes, lay down hay thick enough and very little will grow through until it starts to break down.
Finding bad hay when you anticipated good is almost tear worthy, even just a few bales, but bad hay has lots of value too.
Goats or cattle as DB suggested. I loff it for speeding up the composting of my manure piles, and keeping them tidy and from looking like a hill o' crap, I blanket each pile in old hay.
If you want to kill off growth in an area you can spread it down thick. I love it in the garden, especially container gardening, I put a thick wad of old hay at the bottom of my smart pots. It helps wick up moisture from the ground and keep the pots from drying out. I also use bad hay as a mulch where I don't want weeds to grow, but I have to mulch over that as by the end of the growing season I'll have a carpet of grass from the hay seeds instead.
And thats the final use I love, if its a seedy cutting, I spread it in the pastures where the grass is sparse or there is little or no thatch, to spread seeds and leave some organic material to help hold moisture.