One cannot arrest and detain a trespasser as a "citizen's arrest" in Virginia in most circumstances. In fact, if one does, he could be criminally charged with Abduction!
It is best to call the police to report a trespasser.
Take pictures then call the cops.
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I have a hard time envisioning a scenario wherein this would go well for you. Best to just post a sign and/or contact the police if it happens repeatedly.