I get the error "Cannot delete member who owns forms, memos or actions". 

In Samm's architecture it is expected that the owner of forms, memos and actions is a member of the project that they reside in. To remove the project member, while they are the owner of forms/etc, would go against that, which is why an error occurs. 

Options for this situation are:
(A) Change the ownership of all of their forms/etc to someone else. After that you will then be able to remove them as a member of the project. Note: If the forms are closed then they will need to be re-opened before they can be edited (using the web app).
(B) Edit their project membership from "Send" to "Read", in which case they won't be able to edit/create/send forms but could still view them.
(C) If they are no longer working with Samm or for your organisation then you could make their account inactive (in the web app, via Admin => Users).
(D) On their device, you could unsubscribe from the project (but then they could re-subscribe if they wanted to).
(E) If option (A) is onerous and options (B) to (D) unsuitable then contact Samm support to explain the situational requirements and ask about getting assistance and whether a suitable option might be to re-create the user's account.