Add the -x command line option to eXclude test groups from the test groups to be executed.
It should work just like the -F option. -x is followed by a list of test groups to be excluded. Only the test group name is needed, not the fulll path. The .testGroup extension is added if it is missing. Case is insignificant in the test group name.
To exclude the RexxContext.testGroup you could use this command line:
testOORexx -x rexxcontext
Nothing prevents you from using a complete file name if you wish, this works as well:
testOORexx -x ooRexx\base\class\RexxContext.testGroup