Spring's maven external repository does not contain these artifacts anymore (licencing issue?).
You should download the desired release from adobe, unzip and jar -x
the contained war file and at last you must drop all the flex* jars into your local repo with:
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-common -Dfile=flex-messaging-common.jar
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-core -Dfile=flex-messaging-core.jar
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-opt -Dfile=flex-messaging-opt.jar
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-proxy -Dfile=flex-messaging-proxy.jar
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-remoting -Dfile=flex-messaging-remoting.jar
mvn install:install-file -Dpackaging=jar -DgroupId=com.adobe.blazeds -Dversion=4.0.0.14931 -DartifactId=blazeds-rds-server -Dfile=flex-rds-server.jar
or upload each in your private maven repository (i.e. artifactory or nexus).
Note: replace the version with the one you've downloaded
thanks to pledge's post in flex forum