Then just switch to simple recovery, switch back to full immediately, run a full backup and restart the log backups after that. Providing there's nothing else preventing log reuse, that will leave most of the log inactive and the log backup right after the full will just have the log records since the full backup, after that they'll, as normal, have all log records since the previous log backup.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability