尝试一下:

In [98]: pat = r'\b(?:{})\b'.format('|'.join(remove_words))

In [99]: pat
Out[99]: '\\b(?:abc|def|pls)\\b'

In [100]: df['new'] = df['string'].str.replace(pat, '')

In [101]: df
Out[101]:
               string              new
0  abc stack overflow   stack overflow
1              abc123           abc123
2          def comedy           comedy
3          definitely       definitely
4            pls lkjh             lkjh
5             pls1234          pls1234