We demonstrate the use of spatial filtering algorithms for radio frequency interference (RFI) mitigation in conjunction with a focal plane array of electrically small elements. The array consists of a seven-element hexagonal arrangement of thickened dipole antennas with 1600 MHz designed center frequency backed by a circular ground plane at the focal plane of a 3 m parabolic reflector. Rooftop-mounted signal sources were used to simulate a weak signal of interest at boresight and a strong, broadband interferer in the reflector sidelobes. Using an adaptive beamformer, the amplitude of the interfering signal was reduced sufficiently to recover the signal of interest. For an interference to noise ratio of 15 dB as measured at the center array element, the interferer was suppressed to the level of the fluctuations of the 10-s integrated noise floor (the minimum detectable signal level was interference-limited and no longer decreased after 10 s integration). Similar cancellation performance was demonstrated for a nonstationary interferer moving at an angular velocity of 0.1° per second. Pattern rumble due to beamformer adaptation was observed and quantified. For a moving RFI source, the degree of pattern rumble was found to be unacceptably large in terms of its effects on the maximum stable integration time and receiver sensitivity. An array feed with more elements together with specialized signal processing algorithms designed to suppress pattern rumble will likely be required in order to use adaptive spatial filtering for astronomical observations.