What can i do to stop my cats from eating the flowers in the vase?

my mom cant buy flowers anymore because both of our cats eat the flowers from the vase. theyll climb onto the vase to eat them, which ofcourse usually knocks over the vase. they always do it when we get flowers, theyre both female and tortuise shell cats (in other words, they wont listen to us) so how do we fix this problem?