To only get fortunes from homer.dat you simply call fortune as such:
Or, you could delete the .dat files associated with the default fortune files.
From man fortune,
"If a particular set of fortunes is particularly unwanted, there is an
easy solution: delete the associated .dat file. This leaves the data
intact, should the file later be wanted, but since fortune no longer
finds the pointers file, it ignores the text file."