I had the unfortunate experience of trying to remove a ground anchor last year from a rented garage.
Took me three evenings before I found a cunning method to defeat it without sledgehammering the concrete floor of the garage or mullering the anchor with an angle grinder. (no, I won’t divulge the method!) With bikes in place my method would be impossible, though obviously a thieves weapon of choice would be bolt croppers/angle grinder which would make short but noisy work of any chain 🙁
But as mentioned above, they know whats in there now…you need to make it really difficult for them for a while. Personally I’d remove everything of value, hopefully they’ll then lose interest and not bother checking again in future.