Hello Shweta,
best solution is the one described by Anubhav.
Create a custom dynamic control class for metadata and its method to control follow-on document creation based on FOLLOW_ON_PROCESS method from /SAPSRM/CL_PDO_DYN_MDA_HD_QTE class: if at least one PO exists as follow-on doc for Quotation, do not display the create PO button.
Regards.
Laurent.