I imagine an attacker could still use the Veeam UI to delete the files.
If you have access to Veeam UI you can do anything
But, we are taking about Ransomware and deleting/encrypting backup files. in this case, you must prevent easy acces to this files.To do that, is the best way using devices with no directy access (CIFS/NFS/local file system), but external boxes, Linux (SSH, Data mover), ExaGrid (Data mover), DataDomain (DDbost) and HPE StorOnce (Catalyst). Linux is cheapest solution.
DataDomain have feature Retention Lock, which prevent deleting/changing files until expiration period. Retention Lock work only wtih NFS/CIFS not with DDbost.
The best way to prevent delete your backups and have it localy is still old fashioned tape libraries