df = pd.merge(df1, df2, how='left', indicator='Exist')
df['Exist'] = np.where(df.Exist == 'both', True, False)
df = df[df['Exist']==True].drop(['Exist','z'], axis=1)
import pandas as pd
USERS = pd.DataFrame({'email':['a@g.com','b@g.com','b@g.com','c@g.com','d@g.com']})
print (USERS)
email
0 a@g.com
1 b@g.com
2 b@g.com
3 c@g.com
4 d@g.com
EXCLUDE = pd.DataFrame({'email':['a@g.com','d@g.com']})
print (EXCLUDE)
email
0 a@g.com
1 d@g.com