Inspired by 20% monthly mining vs 5% monthly trading

What would prevent me from investing $1,000 for a month, pulling out, investing with a different scammer, rinse & repeat?

This question is not specific to the inspiration question but Ponzi/Madoff/investment scams in general. What stop-gaps do scammers implement to prevent me from scamming them?

Are there legally binding stop-gaps such as contractual clauses which state something like "Must invest for 12 months minimum. Early withdrawal forfeits all interest and costs 20% of principle."

Does anyone have samples of real scammer investment contract clauses which could be considered as stop-gaps?