I live in a region where you have trails that are open any time of the year, some of the other trails are usable throughout the year except rainy (monsoon) season.
There are also rare cases of the trails with sharp slopes regarding which trekkers are advised against using them in Summers (due to extremely dry weather causing soil and grass drying which turns into a extremely difficult section to pass through). In our region, people typically have got familiar with marking of trails by best natural ways instead of signboards (which are very rare).
What are the best possible ways to indicate a 'closed' trail?
To start with, what I have seen for decades now, villagers usually put a huge branch of thorny bush on the trail so that no cattle (or a trekker) continue to walk.