It is many years since I played this game, and I no longer have a computer which can handle it, so I'm going from a very old memory.
I think you have to "talk to" the hireling (press 5, 6 or 7 on the keyboard, whichever is the hireling's appropriate number) and the display includes an option to dismiss. You might also be able to talk to him or her by clicking on her portrait in the adventure screen – if left-click doesn't work, try right-click.
I hope this helps! Please can you get back to me to let me know what happened? Thanks!