[Any way to disable DSM updates completely?]

Yes, Sorry for thinking I may have found a way to stop the updates and the notifications.

I agree it does not work properly.

If you fool the DSM into thinking it is up to date yes it stops the updates and flags, but any apps installed then flag as requiring an update.

[Any way to disable DSM updates completely?]

Just noticed a minor problem with the fix I added above.

Packages will not install.

Workaround is to change the 6.1.1 version file back to the 5.1.0, install the package then change them back to the 6.1.1

[Any way to disable DSM updates completely?]

Just found a really easy way to totally stop the updates and remove the flag.

There is 2 files /etc/VERSION and /etc.defaults/VERSION these contain the following text

majorversion="5"
minorversion="2"
buildphase="hotfix"
buildnumber="5592"
smallfixnumber="0"
builddate="2015/06/29"
buildtime="17:28:56"

 

replace them with 

 

majorversion="6"
minorversion="1"
productversion="6.1.3"
buildphase="hotfix"
buildnumber="15152"
smallfixnumber="1"
builddate="2017/07/19"
buildtime="16:29:13"

 

This appears to stop the updates completely.