Same issue. I've tried using an old version, as you mentioned, but it gets overridden nonetheless. I even tried going inside the app and removing an "iconengines" folder, to no avail. It drives me nuts! These hardcoded online/offline icons are unbelievably out-fashioned. The only workarounds that occur to me are to switch to iSoul's deprecated 2012 build, and run SoulseekQt on Parallels, neither of which is particularly appealing.
Please! Add an option to remove the online/offline display!