Female cats can have kittens by multiple Toms. What happens is that she mates with multiple males and sperm from different males fertilizes different eggs. Note that just because a kitten looks different, does not mean anything. Cats can sometimes have kittens that look much different, even though they are all from the same sire.
The only way to prevent kittens without altering her body chemistry is to confine her whenever she goes into heat so no intact male can reach her.
There may be a way to abort a cat once she becomes pregnant, but it would involve toxic substances or risky medical steps, like an introduced infection. One mild toxin that has been used for contraception is Queen Anne's Lace, which is kind of like an ancient form of Plan B. The seeds are steeped as a tea and it must be drunk without fail within 24 hours of conception. Even so, it is not reliable.