Changeset fe49adb8ec1c (Aug 14) added ACE's `trayicon_blink` whose description is: _If False, Gajim will display a static event icon instead of the blinking status icon in the notification area when notifying on event._
You can wait for a release that includes that change (I did think 0.15.1 included it) or run Gajim from source.
Running from source is trivial, given that you don't need to build the code. Just:
$ hg clone
$ cd gajim
$ ./launch.sh
Don't miss optional runtime requirements.