This is a common problem. Spanish broom grows much larger than the space often allotted to it. You can prune it. I often see it pruned by shearing or just cutting back. In time this regrowth often dies or declines and look unattractive.
Rejuvenation pruning is a better way to prune Spanish broom.
Rejuvenation pruning is accomplished by pruning one or a few of the main stems to within a few inches of the ground. This should be done before growth begins in the spring. These rejuvenated stems will bloom prolifically as they mature. At the end of the cycle, you will be ready to remove those which re-grew after the process began, starting the cycle again. This is a good way to keep the plant size appropriate for the space allotted to it.