I would say "Putting a padlock on a bag is useless" or "Padlocking a bag is pointless." "It serves more as a deterrent than for its intended purpose" or "It's more use as a deterrent than as an actual lock." Something like that.

Hi there Sean! I would say: 'Having a padlock on your bag is useless.', or if you are being more informal you could say 'A padlock on your bag is useless.' Whenever we have a singular countable noun such as 'padlock', there must be some form of determiner, article or possessive pronoun before it. It must have some form of a / the / this / his / her etc. 'Bag' is also a singular countable noun, so here we also need to put something before it. 'Your' is probably the best option here. The second part of the sentence I would say is correct. I would also say that if the padlock is a deterrent then it is not useless, but the sentence is still grammatically correct! Hope this helps! Let me know if you have any questions