Regarding the newly-announced immigration rule which may see “overstayers” of more than 90 days blacklisted, i.e. banned from returning to Thailand, for 1-10 years, the Bangkok Post reported on July 30: “Foreigners overstaying their visa in Thailand likely will face bans of up to 10 years starting August 25.” This is based on a quote from an immigration official who reportedly told the Bangkok Post: “[The new rule] is expected to take effect on August 25 after being approved by the [interior] ministry.”
The Nation, on the other hand (based on a quote from an immigration division spokesman), reported on July 31: “Starting from August 29 [!], the ban will be imposed on any foreigner found to have overstayed his or her visa or visa-exemption period.” But regardless of whether it’s August 25 or 29 – overstayers have been warned repeatedly to leave the country and clear their overstay a.s.a.p.
Until now, tourists who “overstayed” their visa were usually simply fined 500 Baht/day up to a maximum of 20,000 Baht. According to the new proposed rules, foreigners who overstay their visa for 90+ days would be prohibited from re-entering Thailand for 1-10 years, depending on the length of their overstay.
UPDATE – As of 2015, the new rules have not yet been approved and are not being enforced.