In which year, did this earliest guest star in 1999 for SpongeBob SquarePants stop providing the voice?
SpongeBob SquarePants (season 1):  Among the first guest stars to appear on the show were Ernest Borgnine and Tim Conway voicing the superhero characters of Mermaid Man and Barnacle Boy, respectively. Mermaid Man and Barnacle Boy:  Mermaid Man was voiced by Ernest Borgnine from 1999 to his death in 2012, and Barnacle Boy is voiced by Tim Conway.
2012