Which Membership Provider is your site using?

I've just noticed this too that we had a lot a records in this table.

I think these records were created when using the Marketing module.

If you are using another membership provider then replace the provider name and try.

UIUserProvider UIUserProvider => ServiceLocator.Current.GetInstance<UIUserProvider>();
UIUserProvider.DeleteUser("EPi_AspNetIdentityUserProvider", "userName", true);

Aske, You will have to be very careful here. We need to understand what is tblSynchedUser in EPi is, Have you moved your Identity Provider? Those are your organization users who can log in to your website.
tblSynchedUser store claims, I am not sure data in tblSynchedUser even come under GDPR. If you are a DXP customer, you can raise a support ticket also. EPiServer cares about GDPR issues.

Hi

We are using OpenID Connect, implemented more or less following this guide https://world.episerver.com/documentation/developer-guides/CMS/security/integrate-azure-ad-using-openid-connect/ 

This means we don't have a built in membership provider.

@Sanjay Kumar I tried your approach, but i get an exception stating "Default Membership Provider must be specified.", which i guess makes sense.

@K Khan I'm not so worried about the data in tblSynchedUser, since the source of truth is our active directory. However, the data lingers after a user has been deleted in our AD - that's why i need a way to clean it up.

ISynchronizingUserService

is used to synch users, Don't think so any OOB API can be used to delete claims also, You might need a custom service here, starting point could be if you could us some disassemble and have a look at SynchronizingUserService.

These are the following sps I can see in CMS DB
netSynchedUserGetMetadata
netSynchedUserInsertOrUpdate
netSynchedUserList
netSynchedUserMatchRoleList
netSynchedUserRoleEnableDisable
netSynchedUserRoleList
netSynchedUserRolesListStatuses
netSynchedUserRoleUpdate