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This article should help: https://www.gardeningknowhow.com/ornamental/foliage/yucca/caring-for-and-landscaping-with-yuccas-outdoors.htm

To kill it, cut it down as close to the ground as possible and then paint the fresh cut with concentrated Round-up. Keep an eye on it and if you see any shoots, you can repeat this process.

If it is rooted, you can water when the top inch of the soil is dry. If it is not rooted, place it in a well draining rooting medium and water frequently.

It sounds like it may have root rot. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm

You don't encourage yucca to bloom. They only bloom when they're ready to. If the yucca is planted in the ground, it's more likely to bloom than container-grown plants kept indoors. However, it will still be awhile before it is mature enough to produce blooms. In fact, with some yuccas, it can take years before you see any blooms. My advice...be patient.

Yuccas are fickle with their blooming, especially if they are young. They may skip a year or two in between blooming as blooming can take a significant amount of energy out of the plant. I would give it some fertilizer to help it bring back its energy reserves and be patient.