You need to provide more details about your data before detailed help can be provided. You say you are using Excel. Do yo mean that you have imported data from Excel into R? If the data have been imported, do you need to remove a column named last_name? If that is the goal and your data set is named DF, you can remove the column named last_name with the command
DF$last_name <- NULL
If that does not solve your problem, please explain more about what your data are and what you need to do.