How do I stop my iMac from looking at other networks?

there few other home networks can see house , apparently imac wants see them many times waking sleep either not connect mine or when search mine not top.

home network (an apple airport extreme) marked preferred. can make never another? how can remove others has seen?

 

go to: system prefrences > network > wifi/airport > advanced.
there can change order connects networks , remove networks has connected to.

 

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