I implemented following:

1. The allowed period between two posts is 60 seconds. I haven't tested this one, so please let me know if this is not working.

2. New members can post only 4 messages with 24 hours of their registration. I hope this is enough for genuine members and for spammers this is not enough.

3. No member can post two messages with the same title. This will disappoint the spammers. For existing users, it will happen that accidentally if you give the new topic the same title you have given a year ago, it will redirect to the old topic!

Hope this will help us a little bit.

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A Proud Indian Canadian