In a nodes configuration, choose to specify environment variable. Add a name=value pair, but leave both fields blank. Hit save.

When a job is run on the node, the environment will be setup with an empty environment variable:

Last Built Revision: Revision 908d8e80aeb1af9c4a76f050e556e1c19d1c1222 (origin/master)
Checkout:2010.07.03-alm-continuous / /home/buildslave-1/workspace/2010.07.03-alm-continuous - hudson.remoting.LocalChannel@55aef45
GitAPI created
Env: =
Env: BASE_URL=http://dsc-engbld-03:8001/slm
Env: BUILD_ID=2010-06-24_14-14-33
Env: BUILD_NUMBER=15
Env: BUILD_TAG=hudson-2010.07.03-alm-continuous-15
Env: BUILD_URL=http://alm-build:8080/hudson/job/2010.07.03-alm-continuous/15/
.....

The Env: = will cause issues with the maven surefire test runner if your job is setup to do something like "mvn test"