[R] remove

[Val]

Hi all,
I have a big data set and want to  remove rows conditionally.
In my data file  each person were recorded  for several weeks. Somehow
during the recording periods, their last name was misreported.   For
each person,   the last name should be the same. Otherwise remove from
the data. Example, in the following data set, Alex was found to have
two last names .

Alex   West
Alex   Joseph

Alex should be removed  from the data.  if this happens then I want
remove  all rows with Alex. Here is my data set

df <- read.table(header=TRUE, text='first  week last
Alex    1  West
Bob     1  John
Cory    1  Jack
Cory    2  Jack
Bob     2  John
Bob     3  John
Alex    2  Joseph
Alex    3  West
Alex    4  West ')

Desired output

      first  week last
1     Bob     1   John
2     Bob     2   John
3     Bob     3   John
4     Cory     1   Jack
5     Cory     2   Jack

Thank you in advance