Visitors to a beauty spot near Bristol have littered at the site and even "defecated in the bushes", leading to its closure.
Warleigh Weir in Somerset shut today (May 23) and will remain closed "for the forseeable future".
A statement said the announcement came with a "heavy heart" but that four reasons had contributed to the decision.
There were detailed as: people leaving rubbish, people coming to Warleigh in cars, people not respecting social distancing, and people "defecating in the bushes".
A date for reopening was not given but the statement added: "When it is re-opened we hope to see a major shift in attitude and behaviour, as well as other site users engaging with those who behave poorly."
They said cars were "not welcome" at the popular swimming spot, which is about half an hour from Bristol on the other side of Bath.
Metal fences have now been put up to block the entrance and stop visitors from accessing the weir.
Landowner Johnny Palmer has posted a video on YouTube explaining the decision in more detail.
He said: "It's quite a sad day for me personally. After two years of saying it and threats, I've had to finally fence off the island.
"A lot of people are going to be really angry about this - I am as well, I don't like it at all."
Regarding people leaving human mess behind, he said: "If you've got to go, whatever, all I ask is that people behave as well as their dogs behave and pick it up in a bag. It's not much to ask.
"Don't poo in the bushes, it's disgusting."
Mr Palmer said people had been parking along the A36 and he feared someone would "get killed" if the problem persists.
He added: "The cars are on the edge of the road, I've seen children running across the road, there's trucks going past at 60mph.
"If someone did die, I would feel a sense of responsibility - cars are not welcome."
He said it was "not okay" to leave rubbish there and that the council had installed bins to reduce the problem.
For news tailored to your local area, powered by In Your Area:
Mr Palmer said the problem was a "massive public health hazard" and that he hoped the fences could come down "within a few weeks" as long as people listened.
The swimming spot was also temporarily shut in April after people were seen flouting the lockdown rules.
Earlier this week, Somerset Live reported how the A36 had been 'heaving' as people visited the weir.
