Not knowing what your set up is makes it a little difficult to answer. Clove is supposed to drive ants away. It worked for me years ago. You can place some cloves in a netted material, panty hose work really well. Just cut a piece, place some cloves inside and tie it shut. I wouldn't place it in the cage or anywhere that the bun could possibly eat it but if you have somewhere you could safely place it such as laying a board on top of the cage and sitting it on that may work. I'm not sure though if clove could be an issue with buns so you may want to do some research on that first.
You could also sprinkle some Diatamacious Earth (DE) around the outside of the cage. You want food grade DE. If it is not food grade then it contains pesticides which are harmful but food grade is safe even for eating. It is a very fine powder so I would just sprinkle it around the outside of the cage.