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crippled app for a "bad attitude" ? don't try that with any clients who have any brains

the problem with crippling apps is that what you have is a contract claim against a non-payer ... the law provides a remedy for you but it isn't self-help and intimidation; you sue for non-payment; remember that some clients consider that they have a defense to your claim and they are entitled to assert it

You might instead write up a license which stipulates that in the event of non-payment client agrees that the license to use the software is terminated and take it from there ...