How can I remove a domain from the portal?

A domain can NOT be removed from a portal in Yuki.

A user with the 'Portal administrator' role does have the ability to cancel a domain's contract as of the current month or the following month.

After the term of the contract expires, Yuki automatically changes the status of the domain to 'Inactive'. An inactive domain remains visible in the overview of all domains. However, access to the inactive domain is restricted for all users, including a user with the role 'Portal Administrator'. Thus, no more data can be viewed, changed and/or processed in the domain.

For a detailed description, see article Archive domain or terminate contract of domain.

A domain name can only be used once in Yuki. So a new domain will need to be renamed. 

You can transfer a domain by exporting the documents in the 'old' domain and then uploading them again in a new domain. The documents will then be invoiced again by Yuki.