Never from a normal table or customer. Taking good care of people is how I made my money.
Creepy men: There are a lot of them out there. Not talking about lonely men that just want to chat, but the creeps who make inappropriate comments and/or try to take up all your attention/keep you at their table for long periods of time as a weird means of control.
Drunk people: cutting someone off is awkward at best, and it can become violent and abusive. A neutral cutting off of someone is to ignore them by avoiding eye contact and generally not looking their way, and telling your coworkers not to serve them. But, if they really try to get your attention, you gotta go over and tell them that they will not be getting served any more drinks.
The only other instance I can think of ignoring someone is if they have been assholes throughout their meal, have paid their bill, and are squatting at the table.
Of course it all depends on the level of dining/restaurant.
Edit to Add: And some waiters just suck at their jobs and will ignore tables.
Despite what many people think, it is not an easy job to do well. Ive trained enough people over the years to know that some just cannot handle it. You have to be excellent at reading people and multitasking and properly prioritizing tasks, along with many other skills.