How can I get dried urethane out of a brush?

Slowly, carefully, holding the brush by the tip of the handle, walk to the trash can drop the brush in, and give it a respectful farewell. Forgive me that.

So many supplies and materials we use in the process of doing a job, end up as you describe. Hopefully you used a bargain brush. With no offense at all, had it been a $30 Purdy, you would have known the steps to clean it to useable, before ever dipping it.

Consider it a learning experience...part of ones education in DIY, and hopefully an alert about what steps to take next time.

BTW SPAR is often associated with VARNISH,,,not at all A POLY material/substance.

The only thing you can do is toss it out. Urethane is not meant to be disolved once it has dried. you could try paint stipper but a new brush will cost less.

You can go to a hardware ot paint store and buy a packet of Restore A Brush. Follow the directions on the packet. Paint thinner, varsol or mineral spirits will not do a very good job.

Try lacquer thinner. Believe that will work. Doesn't cost that much to try. If the brush didn't cost very much, maybe just discard it. It it's an expensive brush, try the thinner.