Cut the sirloin steak into 1-inch cubes and place them in a large bowl.
Add the olive oil, soy sauce, balsamic vinegar, garlic powder, onion powder, paprika, black pepper, and salt. Mix well to coat the steak evenly.
Cover and marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
While the steak is marinating, soak wooden skewers in water for 30 minutes to prevent burning on the grill. If using metal skewers, skip this step.
Preheat the grill to medium-high heat.
Thread the marinated steak, bell pepper, red onion, and zucchini onto the skewers, alternating between the ingredients.
Place the skewers on the grill and cook for about 10-12 minutes, turning occasionally, until the steak is cooked to your desired doneness (medium-rare is about 135°F).
Remove the kabobs from the grill and let them rest for 5 minutes before serving.
Serve hot with your favorite dipping sauce or side dish.