Weigh the pros and cons of allowing others to use your cave. Sure, some visitors telephone at odd hours, park their cars where they shouldn't, or leave trash behind. But the majority of cave enthusiasts are responsible persons who respect landowners, their rights and wishes, and most importantly, the cave. Personally, I'm very tough on trespassers but friendly to responsible visitors who abide by the rules I set. On the other hand, if trespassing is out of hand, fragile formations are in danger, or endangered species are being disturbed, close the cave temporarily or permanently.
Gates are quite effective if built from a proper design.
Blasting an entrance closed is not good because it closes the animals' entrance too. Support information gathering at your cave. If you are approached by a state agency, an environmental or conservation organization, or a recognized caving organization to conduct scientific studies, mapping, or exploration of your cave, grant them permission. Valuable information may be obtained on cave wildlife, geology, and hydrology of that area. Information may be kept confidential, at your request, and you give up none of your rights to control cave access.