Identifying himself as fictional characters such as "Lord Voldemort", an insurance agent sent anonymous threatening messages to some of his clients after they cancelled policies they had bought from him.
Myanmar national Ye Lin Myint, 36, who had worked for Prudential, was also unhappy as several others had not turned up for their appointments.
Yesterday, District Judge Marvin Bay said the Singapore permanent resident's offences showed a "remarkable degree of planning", before sentencing him to two years and five months' jail.
Ye Lin Myint pleaded guilty to five counts of criminal intimidation and eight harassment charges.
Thirty other charges for similar offences were considered.
The court heard that he targeted 33 people in all, including his former clients and their neighbours.
He sent them threatening letters and e-mails from August to September 2017.
In July 2017, he went on a portal called Protonmail to create e-mail accounts under the names "Lord Voldemort" - the main antagonist from the Harry Potter novels - and "Dr Bruce Banner", who is comic book character Hulk's alter ego.
Deputy Public Prosecutor Thiagesh Sukumaran said that with Protonmail, netizens do not need to key in their personal information to create accounts on the platform.
He added that Protonmail also does not keep any IP logs which can be linked to the e-mail accounts.
These measures allowed Ye Lin Myint to remain anonymous, the court heard. He also created a bitcoin wallet to receive and transfer the cryptocurrency.
The DPP said: "After he created the Protonmail accounts, the accused also created letters and e-mails... which threatened that harm would befall his clients and their families if one bitcoin was not transferred to the accused's bitcoin wallet." At the time of the offences, one bitcoin was equivalent to about $6,600.
Ye Lin Myint later decided to harass his victims' neighbours to exert additional pressure on them.
His victims alerted the police and officers arrested him after they conducted an extensive investigation.
The Straits Times understands that none of his victims transferred bitcoins to him. He is out on bail of $15,000 and was ordered to surrender at the State Courts on Feb 15 to begin his sentence.