I think the only way to get rid of the client lease on a home grade router like this is to either reset it or power cycle (unplug power - plug it back in a few seconds later) it.
Try power cycling it first - that will prevent you from losing your config. If that doesn't work, only then reset it but that will kill all your settings. Power cycling does the trick on every home router I've ever seen. Let me know if that works!