The reason you are getting error as you are passing a string in "IN " clause and run the sql directly.
Instead use dynamic query like
When you use in clause, then there must be a possibility of deleting rows for multiple ids.
So pass the userid like "1.4.7" from C#
ALTER proc [dbo].[DeletUserFromAdmin]
@userid vrachar(max)
as
declare @sql varchar(max)='';
set @sql = 'DELETE FROM [dbo].[Users] WHERE usr_Id in (' + @userid + ') ';
execute (@sql);
Even you can use parameterized dynamic query for getting rid of sql injection.
Hope this helps.
Cheers