In recent weeks, I have found many cigarettes and even empty beer cans outside my place.
I live on the ground floor which is far away from main road and far away from pier.
The flat beside mine is empty.
I guess someone has treated my garden as a regular smoking and drinking place.
Really hate to clean up those cigarettes and feel not very safe.
What should I do with it....? Would it help if I contact the police?

Interesting... is there somewhere to sit that may attract the drinking smoker? If so, why not leave a notice on the sitting place saying that this is your garden and you dont want fag ends left in it / bugger off etc..? Also, get rid of any sitting .place

Is it a drinking smoker or a smoking drinker? How do you tell?

Calling the cops won't do a thing, the cops are useless, they will probably tell you that no one is trespassing and refuse to take a report. If you go the police route be sure to call 999 so it is logged in to central dispatch and it will at least get some followup.

I've known people on Lamma to make a report to the police about crimes much more serious than this. When returning the next day the same police officer will say he know nothing about it and no report has been made.