As mentioned in the linked answer, the method String#+(other: Any) is added to the String class with compiler magic, rather than with an implicit conversion. As such, it isn't related to the automatic import of Predef._.
The same applies to Int#+(x: String), and corresponding method on the other value types.
However, there is another String concatenation method that is added by an implicit conversion in Predef. x + "2" is treated as Predef.any2stringAdd(x).+("2"). By explicitly importing Predef on the first line of your file, you can rename unwanted members to _, disabling them.
import Predef.{any2stringadd => _, _}
object Test {
object A
A + "20" // error: value + is not a member of object Test.A
}
I don't think that this works in Scala Scripts or in the REPL. There is also an unsupported option, -Yno-predef, to turn of the automatic import globally.
Related: SI-1931
