Radio signals could definitely interfere with LIGO, and show up with consistent time delays corresponding to some sky position. There are two defenses against this:

1. At the LIGO observatories there are numerous auxiliary sensors, including radio receivers, that are used as "vetoes" for candidate gravitational wave events.

2. For binary black hole or neutron star inspirals, the gravitational wave waveforms are well-modeled using numerical general relativity. The fact that the observed detection matches the prediction so well lends additional credibility to the detection.

Regarding #2, we are of course hoping to detect something unexpected, with an unknown waveform. In those cases we'll rely on the veto channels to ensure that we have not detected contamination from a radio event.