Well it depends on which province you're in. There's at least two places I know in Theren which aren't in the guild and have reputation buckets. I'm drawing a blank if there's anything like that on the islands, hib, or ilithi. I'm fairly sure one is not in Zoluren though.

Other options include:
1) Doing tasks for an NPC associated with the thief guild. Obviously this has to be a non-guildleader since you can't get into the guild, but there are other NPCs out there.

2) Staying off the wanted boards and time. Turn in yourself in the province. Make sure you're not wanted anywhere else in the entire province (some places, like the islands or clans, have separated justice systems that you need to check to make sure that you're not wanted). Then wait. Over time, your reputation will slowly shift towards neutral, which can be monitored using the beggar.

Nikpack