I started creating a patch to implement this feature but I got stuck (I am not a developer)
What I started can be found here: http://pastebin.com/raw.php?i=ZixnJ9Ut
As far as I can tell the logic needs to be implemented to lock or not lock the release builds but everything else is there (I think).

Code changed in jenkins
User: James Nord
Path:
src/main/java/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper.java
src/main/resources/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper/config.jelly
src/main/webapp/help-projectConfig-help-numberOfReleaseBuildsToKeep.html
src/test/java/org/jvnet/hudson/plugins/m2release/M2ReleaseActionTest.java
http://jenkins-ci.org/commit/m2release-plugin/82e9e2c2fbdae06a4d04700ed5cf0bdcea9c8ce8
Log:
[FIXED JENKINS-12076] Add the ability to specify the number of release builds to keep.

If the number is -1 then all builds are kept.
a positive number will keep that many sucessfull release builds.
Any other number (< -1 or 0) will cause no release builds to be kept.

Compare: https://github.com/jenkinsci/m2release-plugin/compare/18712bd...82e9e2c

Integrated in plugins_m2release #69
[FIXED JENKINS-12076] Add the ability to specify the number of release builds to keep. (Revision 82e9e2c2fbdae06a4d04700ed5cf0bdcea9c8ce8)

Result = SUCCESS
James Nord :
Files :

• src/main/resources/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper/config.jelly
• src/main/java/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper.java
• src/main/webapp/help-projectConfig-help-numberOfReleaseBuildsToKeep.html
• src/test/java/org/jvnet/hudson/plugins/m2release/M2ReleaseActionTest.java

Code changed in jenkins
User: David Venable
Path:
src/main/java/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper.java
http://jenkins-ci.org/commit/m2release-plugin/456179363429f366e3bacc28bd8290986aae7a0b
Log:
[FIXED JENKINS-12076] The build parameter for the number of release builds to keep was not being saved and thus not used.

Code changed in jenkins
User: David Venable
Path:
src/main/java/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper.java
http://jenkins-ci.org/commit/m2release-plugin/e7633cb63acea8781e55b97d23480e0efd201fdc
Log:
JENKINS-12076 Corrected the name of the public setter for numberOfReleaseBuildsToKeep.

Code changed in jenkins
User: jtnord
Path:
src/main/java/org/jvnet/hudson/plugins/m2release/M2ReleaseBuildWrapper.java
http://jenkins-ci.org/commit/m2release-plugin/a46f076df1c2cbcbb6d2577e27053148644f714c
Log:
Merge pull request #10 from dlvenable/fix-number-of-releases-to-keep

[FIXED JENKINS-12076] Correctly save "number of release builds to keep" parameter

Compare: https://github.com/jenkinsci/m2release-plugin/compare/2e22caafe9cb...a46f076df1c2

How does it work, now I see 'Number of successful release builds to keep' with default value 0, moreover latest release build is NOT locked.

See inline help, or comments above.
If unclear please use the mailing list.

There is no inline help only this message ERROR: Failed to load help file: Not Found

Please raise an issue with your Jenkins version and setup info for the lack of help.

If the number is -1 then all builds are kept. a positive number will keep that many sucessfull release builds. Any other number (< -1 or 0) will cause no release builds to be kept

Issue created and linked, thx for info.