Hi,
I assumed you're using the on-premise version of 365 and you're referring to closing an opportunity. If so, open this aspx file
C:\Program Files\Microsoft Dynamics CRM\CRMWeb\SFA\opps\dlg_closeopp.aspx
and add style="display:none;" to the tr tag just above the sample line below
<tr style="display:none;">
<td id="tdCompetitor"><label for="crmLookup"><loc:Text ResourceId="Web.SFA.opps.dlg_closeopp.aspx_191" runat="server"/></label></td>
**PLEASE NOTE, THIS IS NOT A SUPPORTED CUSTOMISATION SO USE IT AT YOUR OWN RISK**
Eric UNG [Senior Analyst Programmer :: Sydney, Australia]