The burden of proof is on the people who believe they have sighted the Loch Ness Monster to prove it exists with verifiable evidence, however it would be possible to remove all doubt by thoroughly searching Loch Ness. Has this ever been suggest, attempted or completed?
A simple Wikipedia search lists some of the more extensive searches: http://en.wikipedia.org/wiki/Loch_Ness_Monster#Searches_for_the_monster
Some seem rather extensive, involving sonar surveys of the entire loch, submarine expeditions, microphones, etc. The lack of data/evidence is deafening.