Fill in the documentation comments (great, but time-consuming)
Turn off the comment generation (in project properties)
Disable the warning in project properties (in 'Project properties' go to Project properties -> Build > "Errors and warnings" (section), Suppress Warnings (textbox), add 1591 (comma separated list)). By default it will change Active Configuration, consider to change configuration to All.
Use #pragma warning disable 1591 to disable the warning just for some bits of code (and #pragma warning restore 1591 afterwards)
Ignore the warnings (bad idea - you'll miss new "real" warnings)