ID     event
1       2
1       3
2       2
2       2
3       2
3       1
3       5
3       2

ID     event
    1       2
    1       3
    3       2
    3       1
    3       5
    3       2

In [1471]: df[df.groupby('ID')['event'].transform(lambda x: x.nunique() > 1)]
Out[1471]:
   ID  event
0   1      2
1   1      3
4   3      2
5   3      1
6   3      5
7   3      2