If the purchaser has broken a condition of the contract through non-payment, you may be entitled to treat the contract as broken by the purchaser. This is to presume that the non-payment is a breach of a condition and not 'merely' a warranty (in which case only damages are available). Skipping the formalities of communication with the purchaser, the licence should now be treated as voided.
The most recent case concerning ongoing use of a voided licence was in Singapore (bear with me here!) in Novemeber 2008. In this case, it concerned ongoing use of the claimant's GPS map data beyond the contractual period. The judge held that:
Unless the defendant has an explicit condition in the contract with you which allows for use beyond the licence period, the defendant may NOT continue to use it. An implied licence through your constructive knowledge of his having the access to the data is insufficient.
The judge also ordered a injunction to prevent the defendant from continuing to use the licenced software (I won't bore you with the technical names!)
Further, damages were granted on the basis of a statutory breach of the claimant's IP rights(this will vary according to circumstances).
Finally, an account of profits was ordered. The defendant was thus liable to the claimant for all profit made after the termination of the defendant's licence through use of the claimant's material. Again, this is obviously applicable on a case by case basis and I have no way of giving you an exact figure! It is also only available where the defendant has made an unlawful profit and not merely used your material (in that case the statutory award would be made).
I hope that answers your question. If you have any other queries or if I didn't make anything clear, let me know!