Cryptozoologists like to claim that you can’t prove a negative. I respond that 1) scientists don’t deal in proof, and 2) of course, given a specific claim, you certainly can provide evidence that it’s false. If someone is going to make a claim, the onus is on them to provide sufficient specific criteria for the evaluation of that claim.
Here’s an excellent example of how it’s done: Craig McClain dismantles the assertion that the giant shark Megalodon exists. This is a very thorough, point-by-point dissection of the evidence that we should have if there actually were an 18-meter long monster shark prowling our oceans. The evidence shows that, sadly, they all went extinct between 2 and 3 million years ago.
You could make the same sorts of arguments against the existence of a giant hairy ape living in the forests of the Pacific Northwest, or against the Tree Octopus. The True Believers never seem to be dissuaded, though.
It’s hard to make those kinds of arguments against a giant cosmic god, though. Those True Believers have cunningly engineered the properties of their cryptid to be nebulous and evasive; Megalodon at least had specific parameters and predictable properties that allows one to make predictions about what you should see if they existed. Gods have none of that.