Spock-junit5 released
It was not quite correct to completely avoid spock-spring usage because it also provides spock mocks support. Now spock-junit5 is compatible with spock-spring: it detects spock-spring presence in classpath and ignore junit spring extension. This way, spock-spring manage spring context startup, as before, and spock-junit5 manage all additional junit extensions.
Also, there is not a generic @IgnoreJunitExtensions annotation to help in similar situation (or just to be able to temporary disable declared extension).
New @DebugJunitExtensions annotation could help in used junit extensions verification: it shows all registered extensions and when they are called.