What should I do after being denied a 30-day extension on my tourist visa due to lack of space for a stamp?

Why let your passport get to this late stage and then blame the IO for being unfair. Take responsibility for your own actions and when you are down to the last 2 pages or page, order a new passport. Asking them to squeeze stamps in between other ones is not a reasonable request.

Let us all know what happens please. This is first class ignorance... We would all love to know what happens to you! Good and bad please. I hope it all works out for you but please tell, haha!

Do a border run to a country that just stamps and doesn't require a full page visa- i.e. Myanmar or Malaysia, or if you can take a flight Hong Kong doesn't even stamp you at all so won't waste any space at all. Cambodia may also be an option if you get the e-Visa, I have done Cambodia with an e-Visa before simply as I was running out of pages and didn't have room for the normal visa. There is one more extra stamp for the Cambodian e-Visa though, but it's smaller than a visa.

If it's COMPLETELY jam packed, go to Hong Kong. One stamp out of Thailand, one stamp back in, no stamps from HK.

That will fix your immediate overstay issue. I don't have your passport in front of me but you should almost certainly be able to come back as border stamps are a lot smaller than the extension stamp and so they will fit those in if you had nearly enough room for the extensions. In my experience the border immigration are also less picky on this, they will squeeze them in. Then when you get back, apply for a new passport.

I learned the hard way after being denied at Laos border when I thought I had 3 blank pages in my US Passport. The IO told me it says "Endorsements" on it and they will not place a Visa sticker on that page at the border. I got this sorted out by getting a new US Passport at the embassy now with 52 pages for $110.

Despite what others may say... I have know of people who used steam or some other method and removed old Laos Stickers (these Lao stickers always have companion small stamps - so no chronological data is totally missing). But up to you.

New passport is needed...they all say 14 days but in reality you will get it around 10 days ...

Your only option from what you have said is to apply for a new passport. You will then have to pay an overstay fine. If what you are saying is correct you do not even have room for an exit stamp let alone an entry stamp to another country or any room for a TR visa. Pretty sure this is your only course of action. (Would love to know what airline allowed you to travel here in the first place?)

You started the question with: Extension is denied due to lack of space. Did you blank out some stamps in your passport to make space? I hope you did not. If there is no place for the stamps, even when you go back later there will be no space for stamps. If you apply for Extension after expiration of the Visa, Immigration may give you the Extension starting from your last Admitted to Stay date plus overstay fines for the days you are to late.

a Full passport. Only option is to apply at your Embassy for a new passport and that will take about 2 weeks. a Border run will be cheaper than the overstay fines.

If you have a USA Passport where the last pages are marked "Endorsements", then you have no choice but to apply for a new passport. Immigration will only stamp on pages marked "Visas"...