We know that Makkas Dam, the only other makkah that was created by transforming something, rather than summoning it from elsewhere or creating it out of nothing, the waters turned back to their original state after the Makkah.1 Did the same thing occur by Kinnim and Shchin? Did the lice revert to dust, and did the boils dissolve into oven ash? Or did they just vanish into nothingness?
1This can be inferred from Shemos 8:1, which discusses "rivers" - if the blood simply vanished, then there would be no rivers, as all of the previously existing water in Mitzraim turned into blood. And it couldn't have remained blood, as then Makkas Dam would have lasted far longer than its allotted week (see Rashi to Shemos 7:25). Thus, it must have been water. (Others say this fact explicitly.)