  The matched filtering technique is used to search for gravitational wave
signals of a known form in the data taken by ground-based detectors. However,
the analyzed data contains a number of artifacts arising from various
broad-band transients (glitches) of instrumental or environmental origin which
can appear with high signal-to-noise ratio on the matched filtering output.
This paper describes several techniques to discriminate genuine events from the
false ones, based on our knowledge of the signals we look for. Starting with
the $\chi^2$ discriminator, we show how it may be optimized for free
parameters. We then introduce several alternative vetoing statistics and
discuss their performance using data from the GEO600 detector.
