Question: In which year, did this earliest guest star in 1999 for SpongeBob SquarePants stop providing the voice?

<P0-S0> Mermaid Man and Barnacle Boy are fictional superheroes in the Nickelodeon animated television series "SpongeBob SquarePants". <P0-S1>  Mermaid Man was voiced by Ernest Borgnine from 1999 to his death in 2012, and Barnacle Boy is voiced by Tim Conway. <P0-S2>  Mermaid Man and Barnacle Boy were created and designed by Stephen Hillenburg. <P0-S3>  The characters debuted on August 21, 1999 in the series' episode "Mermaid Man and Barnacle Boy". <SEP> <P1-S0> "Help Wanted" is the pilot episode of the American animated television series "SpongeBob SquarePants". <P1-S1>  It originally aired on Nickelodeon in the United States on May 1, 1999, following the television airing of the 1999 Kids' Choice Awards. <P1-S2>  The episode follows the protagonist, an anthropomorphic sea sponge named SpongeBob SquarePants, attempting to get a job at a local restaurant called the Krusty Krab. <P1-S3>  However, he is tasked to find a seemingly non-existent high-caliber spatula because the owner, Mr. Krabs, considers him unqualified for the position. <P1-S4>  Eventually, crowds of ravenous anchovies stop by the Krusty Krab and demand meals. <P1-S5>  SpongeBob returns from his errand, having fulfilled the request of Mr. Krabs and found a mechanical spatula. <P1-S6>  He utilizes the spatula to fulfill the anchovies' hunger. <P1-S7>  SpongeBob is then welcomed by Mr. Krabs as a Krusty Krab employee. <SEP> <P2-S0> The first season of the American animated television sitcom "SpongeBob SquarePants", created by former marine biologist and animator Stephen Hillenburg, aired from May 1, 1999 to April 8, 2000, and consists of 20 episodes. <P2-S1>  The series chronicles the exploits and adventures of the title character and his various friends in the fictional underwater city of Bikini Bottom. <P2-S2>  The show features the voices of Tom Kenny, Bill Fagerbakke, Rodger Bumpass, Clancy Brown, Mr. Lawrence, Jill Talley, Carolyn Lawrence, Mary Jo Catlett, and Lori Alan. <P2-S3>  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. <SEP> <P3-S0> In addition to the show's regular cast of voice actors, guest stars have been featured on "SpongeBob SquarePants", an American animated television series created by marine biologist and animator Stephen Hillenburg for Nickelodeon. <P3-S1>  "SpongeBob SquarePants" chronicles the adventures and endeavors of the title character and his various friends in the fictional underwater city of Bikini Bottom. <P3-S2>  Many of the ideas for the show originated in an unpublished, educational comic book titled "The Intertidal Zone", which Hillenburg created in the mid-1980s. <P3-S3>  He began developing "SpongeBob SquarePants" into a television series in 1996 upon the cancellation of "Rocko's Modern Life", which Hillenburg directed. <P3-S4>  The pilot episode first aired on Nickelodeon in the United States on May 1, 1999. <P3-S5>  The show's ninth season premiered in 2012, and episodes of "SpongeBob SquarePants" have aired. <P3-S6>  A feature-length film adaptation of the show, "The SpongeBob SquarePants Movie", was released in 2004; in 2015, a sequel, "", was released.