Just make sure you are logged into an admin user account in OS X & you should be able to drag the trial version to the trash, as MEB said.
You may also want to open your user's Library folder (not the top level Library one) & its Container folder & look for folders in it whose names begin with "com.seriflabs.affinity." If you see any ending with ".beta" (like "com.seriflabs.affinitydesigner.beta") you can also delete them.
