You can edit the Ruby bundle's language grammar to define "words". There's a section called "variable.other.readwrite.instance.ruby", which looks like this:
name = 'variable.other.readwrite.instance.ruby';
match = '(@)[a-zA-Z_]\w*';
captures = { 1 = { name = 'punctuation.definition.variable.ruby'; }; };
Remove the @-sign from the regex and it will behave like you described:
name = 'variable.other.readwrite.instance.ruby';
match = '[a-zA-Z_]\w*';
captures = { 1 = { name = 'punctuation.definition.variable.ruby'; }; };
Note that now syntax highlighting won't catch the @ as well.