Keeping male guppies together is entirely possible and avoids breeding, but requires a larger tank (75+ litres is great) with plenty of plants/decor for breaks, good filtration, and a group of 6-10+ to spread aggression, as males can still chase each other establishing dominance, but usually less intensely than with females around; they need clean water (23-26°C), a varied diet, and shouldn't be overcrowded to thrive.