How can I remove a Filestack file I no longer need?

Filestack has a Javascript method and a REST method for removing files. Both of these functions will delete the Filestack link, Filestack's record of the metadata for the file, and the file itself if it was stored to S3 or another custom storage option.

Here is an example that uses the v3 Javascript API:

var client = filestack.init("AJxHNUUcQHG6ZC0aksrb1z", {
  policy: "eyJleHBpcnkiOjE2ODgxMzkwNTIsImNhbGwiOlsic3RvcmUiLCJyZW1vdmUiXX0=",
  signature: "d42db977580f14653f98b19d41a2c5cc3dcc7cd6db6f0de4cd85db3ee8babe3a"
})

var url = 'https://www.filestackapi.com/api/file/vs6qqpoRQyu438c07LEA'
var handle = url.substr(url.lastIndexOf("/") + 1)

client.remove(handle)

Or using REST:

>>> curl -X DELETE https://www.filestackapi.com/api/file/**HANDLE**?key=APIKEY

You can read more about these two methods here: Javascript API Remove and REST API Remove.