I have just posted a very similar question.
I have no problem tipping (except at buffets) but 18% seems really high. Is 18% mandatory for a party of 6 or more? Can I ask for it to be lowered or removed?
You can absolutely ask to have an auto-gratuity removed. I've never worked at Disney, but I have in the past worked at large, upscale casual corporate restaurants that emphasize excellent customer service. While it was rare for a party to ask to have the auto gratuity removed, I have never heard of an instance where a manager refused to remove the auto grat. I guess it's possible, but I think it would be odd for a manager to refuse to remove the gratuity, especially at a place like Disney where there is such an emphasis on customer service.
Now, the manager likely will ask you why you want the gratuity removed, and if you say that it's too much, the manager may ask if the service was lacking in some way. It shouldn't be a confrontation, but if you are only asking to lower the gratuity for the sake of paying less, you should be prepared to state whether or not you thought the service was adequate. If your server did a good job but you just can't afford the tip, you should say that to the manager so there are not negative repercussions for the server.
As a former server, I'll say that it can be problematic for you if a party asks a manager to have the auto gratuity removed, because depending on how reasonable the management is, it may be perceived that the server is doing a bad job. If the server did a good job, please make sure to say that to the manager so that the server does not later have to try to explain to the manager what he or she did wrong in serving your group.
Of course if your server really is bad, then the auto grat should be removed because the server didn't earn it. But, if the server did a great job, I'd recommend that you leave the 18% auto grat on the bill.
A little bit about serving in the US from a former server:
Serving a large party is a very difficult job. It requires a lot of coordination in terms of timing and in terms of working with other staff. Usually, the servers who have large parties will have to rely more on supporting staff (like bussers and food runners) to help them out in managing service to your table and the other tables in their section. Then the server will have to tip those people out at the end of the day based on their total sales, not based on their total tips. So, regardless of what you tip, that server will still have to pay out a set percentage to each of the supporting staff members at the end of the shift.
On top of that, the server may need to tip out a little extra to supporting staff based on help with a big table in order to maintain a good on-going working relationship with those co-workers. So, many times the auto grat is a way to help servers cover the costs they incur by having to tip out supporting staff based on a higher sales amount. For this reason, I generally believe an 18-20% gratuity is justified, unless the server was inattentive or rude.
If you are not able to afford to tip on a larger bill, then I would recommend that you select more inexpensive restaurants that will allow you tip to the appropriate amount based on the service you receive, regardless of how much the meal cost.
Unlike in other countries, servers in the US do not make minimum wage. In most states, servers make half the amount of minimum wage. That basically means that unless they are earning tips, they essentially make $0/hour because most of the hourly rate goes towards paying out taxes. Therefore, tipping is the only way that servers make any meaningful wage each shift. Based on my experience, the difference between an 18% gratuity and a 15% gratuity on a large check can make a significant difference in how much money the server takes home at the end of the shift. So, I would suggest that you consider that and really assess how good the service was before asking to have an 18% auto gratuity removed.
Not trying to start a great debate here, just hoping to provide some useful insight from the perspective of a former server at a large-scale restaurant.