Jenkins LTS 1.509.4, m2release 0.13-SNAPSHOT, Oracle JDK 1.7.0_25
The "Number of successful release builds to keep" configuration does seem to work. I've configured it to "1", but the no release build is kept.
1. Configure "Number of successful release builds to keep" to "1"
2. Do a release build
3. Do Snapshot builds until the release build is the last build
4. Do yet another Snapshot build and you'll see the release build getting disposed