Use the steel wool that you find in the paint departtment that has no soap in it.
Forgot to say that you should be very careful about using the bathroom-always check to make sure the snake isn't swimming around in the toilet or shower.
The presence of a snake in your walls is a heads-up that there is food in there for them-rats, mice, and squirrels are on a snake's preferred menu and they will go where the food is.
Seal all the holes and cracks you can find with the expanding foam over the steel wool, it's the best way to seal holes being used by snakes (and mice, etc, btw) but you also need to find out and eliminate whatever it is that has attracted the snake, too.
Contact your local extension service for advice, and you might give some thought to having an exterminator out, the pros will be able to tell you what is attracting the snake. If it's rodents, thank the nice snake for the warning and have your wires checked-vermin chew and that can lead to a house fire.
You can buy a can of spray foam. It hardens after you spray it and fills holes. You might want to call a exterminator to find it in case it is poison. I saw on the Animal channel that small poison snakes are worse than big ones because they do not know how to conserve the venom.
In the short term you can stuff some steel wool in the hole. That keeps mice out, and they won't chew through it. I suppose it would work for snakes too.
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