An investor has succeeded in halting Abigail Adams National Bancorp's planned acquisition of a Virginia bank. The bank had until the end of 1997 to close its deal to buy $72 million- asset Ballston Bancorp but could not secure enough votes to win shareholder approval. The campaign against the deal was led by Marshall T. Reynolds, a member of Adams' board of directors. Mr. Reynolds told Adams in October that he intended to vote against the deal. Adams sued Mr. Reynolds Dec. 12, claiming he was unlawfully lobbying other shareholders against the merger. James H. Schropp, attorney for $128 million-asset Adams, confirmed that the deal is off and said the bank plans to proceed with the lawsuit. "The bank regrets that the Ballston transaction was defeated," Mr. Schropp said, "but it intends to continue with the suit, claiming this vote was the result of an illegal proxy solicitation."