Hey Nigel, 

If your card has just one assignee, you can run a updateCard mutation with an empty value in the assignee field_value, like this: 

 

mutation{updateCard(input:{id:123456789, assignee_ids:i0]}) {
  clientMutationId
  card{
pipe {
  id
}
assignees {
    id
  }}
}}

But it’s important to mention here that this mutation replace any value in the assignee field_value.

So if there’s more than on assignee you must run the mutation with the assignee id you need to keep in the card, excluding the rest of assignee ids.

Like this:

My card has two assignees: 
 

{
  "data": {
    "card": {
      "assignees": "
        {
          "id": "654321"
        },
        {
          "id": "987654"
        }
      ]
    }
  }
}

But I want to keep just the 987654. So my mutation will be:

mutation{updateCard(input:{id:392042201, assignee_ids:a987654]}) {
  clientMutationId
  card{
pipe {
  id
}
assignees {
    id
  }}
}}

Hope it helps you :)