The "Trigger/call builds on other projects" build step locks the job if one of the "Projects to build" is deactivated.
1. Add the "Trigger/call builds on other projects" to the build steps.
2. Define some "Projects to build"
3. Activate the check box "Block until the triggered projects finish their builds"
4. Set "Mark this build as unstable if the triggered build is worse or equal to" to "UNSTABLE"
5. Deactivate one of the projects configured in "Projects to build"
6. Run the job
The deactivated project will be ignored and the job continues with the next active project.