Banning by IP address/hostname has always had its problems with banning innocent users along with the spamtards. It's not limited to mobile users. If the IP address is coming from someplace that has no business being on your forum (e.g., a forum on Idaho fly fishing, and the spammer is coming from India), you're not going to lose many legitimate members by banning that IP/host. Otherwise, consider banning on IP address/hostname (if you've checked and have few or no legitimate users there), and lift (remove) the ban after 3 to 6 months. If you have a number of legitimate users there, it's going to be more of a game of Whack-A-Mole.