How to stop web browser offering to sign into Google account
The only solution I am aware of (besides hacking the code itself) is blocking permissions:


Permissions denied
PDroid privacy extension


Regarding PDroid: Currently, only Gingerbread is supported officially but you may find a working patcher for your CM9.1 firmware on XDA. Its Google Play description says: PDroid allows you to block access to the following private data for any application regardless of its permissions: [...] Account auth tokens, [...]

EDIT:
Tried PDroid and found it to work great. 
You need a compatible ROM (CM, AOKP, PA,...) and AutoPatcher (download here) plus either the original PDroid or the PDroid2.0 app (see 1st post/attachment). Note: v2.0 needs JellyBean, use the old one for ICS and below (the original author hasn't updated the app for some time and hasn't ported the original Gingerbread patches. Autopatcher includes both ported original PDroid patches and the v2.0 fork for JellyBean)