I don't know how to adjust the grammar checker for this (Scrivener uses the OSX default), but to clean up double spaces in Scrivener, you first select the text, then choose: Edit > Text Tidying > Replace Multiple Spaces With Single Spaces.
There are a few other goodies in the Text Tidying menu if you happen to import some text from word processor apps that have added unnecessary page breaks and tabs etc.
I want to go the other direction, where a single space can be converted to double spaces. It's one of those silly (stupid) APA style guide rules. Maybe I'll ask Scrivener to add it as a feature.