tzdata (IANA Time Zone Database) to store time zone data, and Daylight Saving Time (DST) information is included in it. The data tells if DST is observed from what date/time to what date/time on a given time zone. Thus, you can't really disable the DST if you're using a time zone that observes DST.
Therefore, to "disable" the DST, you have to choose a time zone that doesn't observe DST, and has same offset with your current time zone. (You can try to find it from Wikipedia's List of tz database time zones)
You also have to turn off Automatic time zone option to prevent it being changed back to the network provided time zone.