Yes, Universities can simply refuse to participate in this deal, but it puts them in a worse position, as previously, the deal was usually made on state level, which afaik no state does anymore. I'm in BaWü and now the choice is either negotiating with MS individually or taking the Bundesvertrag. I think technically they are still considered individual deals per state, but they are effectively the same everywhere ("Campus und School-Rahmenvertrag").

E.g.:

https://www.urz.uni-heidelberg.de/de/microsoft-landesvertrag

https://www.rz.uni-freiburg.de/services/beschaffung/software...

https://www.tu-chemnitz.de/urz/software/mslandesvertrag.php