WhatsApp is imposing 24-hour lock-outs on people trying to access its service via an unauthorised Android app.
The Facebook-owned messaging service said that it had acted against users of WhatsApp Plus because of concerns that the program might cause private data to be leaked to third-parties.
The unofficial app offers extra ways to customise how conversations appear.
Experts say Android users should be cautious about where they download apps from.
WhatsApp recently reported it had 700 million users sending an average of 30 billion messages a day. It currently charges a $0.99 (65p) annual fee to users who sign up to the service after their first 12 months of using it.
According to one app store, WhatsApp Plus had itself been downloaded more than 35 million times since its 2012 releases by a Spanish developer, who called himself Rafalense.
Security consultant Graham Cluley said that figure sounded "surprisingly high", but acknowledged there was no independent way to verify it.
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