Political content is permitted in signatures as part of the balance between freedom to express opinions and maintaining order in the forums. Forum members have used signatures to list their Apple products, to be silly or serious, to promote causes (including the MacRumors Blood Drive), to personalize their online presence, but also to state political opinions. Normally this does not disrupt forum discussions, so it's an overall benefit to allow them. In extreme cases, the rules permit the moderators to change or remove a signature that is causing problems.
The Political News and PRSI forums are separate forums that members are free to visit or to ignore. In the same way, signatures are separate from posts and members are free to display or ignore them.
To suppress the display of signatures, go to your
Preferences and uncheck "Show people's signatures with their messages."
Since signatures are not categorized, there isn't a way to suppress political signatures while allowing others, but of course you can ignore particular users to avoid seeing their signatures.