var list = project.Lists["spisok"];
int num = 17;
Regex regex = new Regex(@"\d+");
for(int i = list.Count - 1; i >= 0; i--)
{
int number;
bool isNumber = int.TryParse(regex.Match(list[i]).Value, out number);
if(isNumber)
{
if(number >= num)
{
list.RemoveAt(i);
}
}
else list.RemoveAt(i);
}