How to remove a complete dataframe column

I am using dataframes and I and I want to delete (remove) a specific
column by name ...
the data frame has 14 columns with several names and only need some.
I tried to overwrite the columns with a single value but that in not very
clean as I need to export the data to file.
The rm command gives me a warning message stating :
< remove: variable "rtu$Ri." was not found >
or there is another method to remove parts of a dataframe?
thanks in advance
Mark Marques