Removing specific rows from a dataframe

I have a data frame e.g.:

sub   day
1      1
1      2
1      3
1      4
2      1
2      2
2      3
2      4
3      1
3      2
3      3
3      4

and I would like to remove specific rows that can be identified by the combination of sub and day. For example say I wanted to remove rows where sub='1' and day='2' and sub=3 and day='4'. How could I do this? I realise that I could specify the row numbers, but this needs to be applied to a huge dataframe which would be tedious to go through and ID each row.