Anti-HST advocate Bill Tielman shows how a sign was altered to send the opposite message to what his side intended. (CBC)

RCMP on Vancouver Island say they haven't turned up any suspects after investigating complaints by an anti-HST group that its signs were professionally changed to hurt their efforts.

Fight HST is seeking to have the harmonized sales tax tossed out in a mail-in referendum and has filed complaints with police in Nanaimo, Ladysmith and Campbell River.

Former B.C. premier Bill Vander Zalm, who has spearheaded the anti-HST campaign, calls the alterations to the signs an "organized effort to subvert the democratic vote."

But Sgt. Sheryl Armstrong with the Nanaimo RCMP detachment said the case is being treated as a mischief investigation, similar to when someone scratches out the name on a realtor's sign and replaces it with another.

Armstrong said officers made no headway through neighbourhood inquiries, checks of surveillance cameras and examination of any evidence.

Fight HST has also reported theft of more than 40 signs to Vancouver police, three weeks after the group alleges more than 100 signs were pinched from the same area.

HST ballots are being mailed to all B.C. residents on the provincial voters' list, and are to be returned by Aug. 5. The date has been extended due to the post office contract dispute that stopped the mail for two weeks.

Results of the binding referendum are expected to be announced at the end of August.