without knowing the type of contaminant it is very difficult to give you a cleaning procedure.
The general procedure would be reversing the column and using your solvent (THF in this case) at 50% of the normal column flow you use for ca. 20 column volumes.
If this does not help you could add some percentages of NMP or DMSO or DMF to the THF and do the same, but pay attention to the system pressure as those solvents have a higher viscosity and therefore give you more pressure, better to lower the flow then.
Thank you, I'm going to try that and let you the outcome .