But some members are bypassing this by using alt accounts to bump their threads. They just some question related to the service or leave a positive reply on the thread. Is there any way to put a check on this ?
As other users have said, you can report them to a moderator and you can actually tag them for alt if you can show valid evidence. I don't know which user you're talking about, but if you don't mind, it's best to tell us about them and which thread they're abusing.
It could in reality be different people who have decided to partner up and bump each other's threads. It will be difficult to prove it's the same person if they are careful. Maybe the registration dates of the alleged connected accounts can shed some more light on whether or not there is a connection.
It must be difficult if they are not the same person but have the same duty to maintain the above thread. But to be honest, I don't know if that can be considered as abuse if they are actually partners.