Is there a protocol for freezing and thawing Bacillus subtilis cells?
While I have never done bacterial stock cultures from spores, I don't think it is necessary, as the standard procedure for making bacterial stocks should work without problems. 

For that you grow your bacteria in liquid culture until you reach the late log phase (so you have a lot of bacteria in your media), take 1 ml of the culture, mix it with 100% glycerol to reach a final glycerol concentration of 50% and then freeze it a -80°C. At this you can store it for long times (probably not indefinitely but for years is possible). 

When you want bring your cultures back to life, take the tube and either use an inocculation loop or a pipette tip to scrape a bit of the frozen culture and plate it on an appropriate agar plate. If you want to scale up, pick one colony from the plate into liquid media.

Since glycerol is a pain to pipette (and can also have sterility problems), I have switched over to DMSO as a cryo-protective agent for bacterial cultures as well. Simply add 15% (end concentration) DMSO into the cryo vials (so 150ul DMSO to 850 of culture), mix and then freeze the cells. In my experience this works as good as glycerol.