You'll have to plug the hole that the standpipe sits on. It should be threaded onto a bulkhead, so you'd just have to find a matching threaded cap/plug. Failing that, you could fit a capped PCV pipe, on the bulkhead. You'd need the fittings to go from PVC to bulkhead though. A google search for "diy durso standpipe" should tell you everything you need to know as far as fittings go.
*OR* you could just use the standpipe as the drain to your sump/wet/dry.