For DSM 7.0 and above:
1. Go to Control Panel > File Services > SMB and click Advanced Settings.
2. Set Minimum SMB protocol to SMB2 to disable SMB1.
3. From Enable server signing, select Disable or Force.
Notes from the knowledge base:
- On DSM 7.2 and above and DSM Enterprise, SMB signing cannot be fully disabled. In Server signing status, the available choices are Force, Client defined, or Disable SMB1 signing only.
- Enabling server signing can significantly reduce file transfer performance.
- If a client does not support the selected signing mode, it may fail to connect.
- SMB transport encryption also significantly impacts file transfer performance.
If you need the older DSM 6.2 path, use Control Panel > Domain/LDAP > Domain > Join domain > Domain Options to access Enable server signing.
Sources used: [1], [2], [7], [11], [13]