It is extremely difficult to do so...
One way is to know her well enough to be able to tell when she is going to be in heat and separate her from the male for a few days when it occures (which proves to be complicated, especially when she is nursing joeys, without even considering the fact that separating bonded pairs is often stressfull for them)
Another way is to have the male neutered before her current litter comes out of pouch (but then, you cannot expect breed her again with the same male in the future, so you would have to either break the pair to introduce her to another male, or stop breeding her altogether)
I know some people managed to recreate a kind of seasonnal pattern by reducing the proteins intake (simulating a winter-like protein-poor period during which the females do not go into heat in the wild), but there's not much details about it, so it sounds difficult to replicate without knowing exactly the amount of proteins needed, nor are there any studies on how it affects the life expectancy of the animals.