To do this hover over the any part of the signature with your mouse and a gray X will appear in the upper right corner of the sig. Click the X and you will receive a drop down which will allow you to ignore that particular user's sig or all sigs.

To reverse that ignore you will need to go to your user profile:

Then EDIT your profile:

Select the Ignore Preferences:

And finally remove the ignore settings for that particular user:

Done and done.