The syntax for deleting a repo is different from the syntax for creating a repo.
To create:
POST https://bitbucket.org/api/1.0/repositories --data "name=mynewrepo"
To delete:
DELETE https://bitbucket.org/api/1.0/repositories/{accountname}/{repo_slug}
Also, the API v1.0 methods are deprecated, so you should use the v2.0 methods.
To create:
POST https://api.bitbucket.org/2.0/repositories/{owner}/{repo_slug}
To delete:
DELETE https://api.bitbucket.org/2.0/repositories/{owner}/{repo_slug}