When you have experimented a lot, there are many things in your notebook and quite often, you are not sure what is there and what not. Zeppelin does not really give you a good tool to do that, so I wrote my own function to clean up the global variables of Python:

def clear(keep=("__builtins__", "clear", 'completion', 'z', '__zeppelin_completion__', '_zsc_', '__zSqlc__', '__zeppelin__', '__zSpark__', 'sc', 'spark', 'sqlc', 'sqlContext')):
    keeps = {}
    for name, value in globals().iteritems():
        if name in keep: 
            keeps[name] = value
    globals().clear()
    for name, value in keeps.iteritems():
        globals()[name] = value

You can then call the function with clear().

Additionally, if you are dealing with libraries, you may want to explicitly remove them with del sys.modules['modulename'].