Vote on own photos is possible for not registered participants

We have as a test a photo contest for the moment.
Members of our photoclub were asked by invitation to upload 5 images each.
The poll is active for the moment and some of the members notified me that they could vote on their own images.
In the poll settings the item to prevent this is activated, so in terms of usage they should not be able to do so.
A registration to PollUnit was not required to upload or vote.
They had to give their e-mail adresses at the upload stage of the images.
The members that I know, until, now that are possible to vote on their own images are the ones that do not have a valid account registered in PollUnit.
If it's mandatory that a account has to be made before voting then please let me know.
Or is there another problem that causes the error?

In the absence of an account, our system can only recognize users from the same device. If the same individual accesses from two different devices, such as a desktop computer and a tablet, they will be perceived as distinct users.

To enhance security measures, it might be advisable to consider requiring a PollUnit account. By selecting the option »Apply restriction also to visitors« in the voting section of the advanced step, submitters will also be prompted to create an account. This approach enables us to accurately identify individuals across multiple devices.

Thanks for your reply.
In the next poll, we will require participants to create an account if necessary.
We have the Excel output for this current poll. This allows us to filter out the points that were added to their own images.
Beste regards,
Eddy