So the selective scamming means that they will stop the withdrawal of large amounts and keep on clearing the small withdrawals ?
Somewhat, yes. Basically, if you're a trusted member of the community they will not fuck you over but if you're a noob who can be silenced, they will scam the shit out of you. Large withdrawals from newbie members or newly created accounts will always be on hold for security/KYC reasons as according to their logic.
Secondly, does it also means that currently there is no site which is 100% secure for sports betting ?
Yes, every site has their flaws and most of them have had scam accusations against them in the past. The sites suggested in this thread are definitely not 100% to be trusted with your money.