You can subset the @data slot which is a plain data frame, but its not always a good idea to access members of an object class via slots, in which case you can convert to a plain data frame with as.data.frame, which drops the geometry and gives back a plain data frame:
bar_vector = as.data.frame(spfoo)[,"bar"]
One advantage of using as.data.frame instead of the slot is that this will also work if your spatial object is one of the new sf classes.