To remove a parked domain, Please follow below simple steps:

Step-1:
Login to plesk panel by using the plesk panel URL: http://yourdomainname.tld:8880 as shown in below image:

Step-2:
Once you have successfully logged in to plesk panel locate "Add domain Alias" in your please panel as shown in below image:

Step-3:
Click on the "Remove Domain  Alias" as shown in below image:

Click on "ok" as shown in below image:
Upon successful removal you will see below error:

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Technical Support,
Triwor