Hi Jenny thanks for posting in the community! From the sounds of it it seems that the document was initially a Lucidspark board which was then converted to a Lucidchart document using our Universal canvas feature. Typically it's not possible to use Guest collaboration in a Lucidchart document but if the original content was from Lucidspark then the Guest option becomes available. 

It's possible that the original owner/creator of the document shared the document with Guest collaborators using the Guest option when creating the link which is why the "Snakey" user appears.

To prevent guest collaborators from accessing the document I recommend that you make a copy of the document and share it with only people you want. You can also also monitor who has access to your document from the "Manage collaborators" section of the Share menu. For more information on this please see this section of our Share with Collaborators Help article here.  

If this doesn't address the issue you are seeing please submit a support ticket via this form and someone from our team will be happy to assist with further investigating. Thank you!