Prerequisites:

• Modded 3DS
• A PC or anything that can use an SD Reader
• SD Card (-reader)

Make sure your 3DS is on the latest available firmware and Luma version. In the Luma settings, make sure external firms/modules and game patches are enabled.

These two files are part of an older revision of the netpass patches, tho the URL has been patched to be invalid

Download the IPS files from here: https://cloud.erdbeerbaerlp.de/s/r76JR76HmJnLXxj

Once your SD Card is inserted, open the luma folder, then the sysmodules folder. Now copy the two .ips files in here.

StreetPass Relay should now be disabled.

To Re-Enable StreetPass Relay, just delete the two IPS Files 0004013000003402.ips and 0004013000002602.ips in the luma/sysmodules folder.