The issue of spammers abusing the power-up system is far beyond just NSFW content. Even though the issue you're facing as a moderator relates to NSFW spammers, the problem with a lot of the users abusing the power-up system scales from a variety of different spammers.

If this becomes more of an issue in the subreddit you moderate, my best suggestion for you is to make an announcement on your subreddit regarding the issue. (Make sure to pin the announcement)

Also, for other issues regarding spammers in general, my advice to you is: Make sure your subreddit has an AutoMod bot put into place. When inviting any type of reddit bot I suggest you take a look at an overview of the bot. You can find lots of bot overviews at r/BotDefense.