Csdrhrt

I currently have the Outlook plugging GpgOL which lets me encrypt/sign emails using an OpenPGP encryption key.

Is it possible to somehow remove the encryption option or somehow hide the encryption option?

If that is not possible, does an Outlook plugin exist which only lets you sign but not encrypt emails?

For signing only you may use Outlook itself, no plugin required.
You may sign a single message or set Outlook to automatically sign all messages.

From Microsoft's article
Secure messages by using a digital signature:

Digitally sign a single message

1. 
   

   In the message, on the Options tab, in the Permission group, click Sign Message.

   
   
   
   
   
   • 
     

     If you don't see the Sign Message button, do the following:

     
     
     
     
     

     • In the message, click Options.

     
     

     • In the More Options group, click the dialog box launcher Dialog Box Launcher button on the ribbon in the lower-right corner.

     
     

     • Click Security Settings, and then select the Add digital signature to this message check box.

     
     

     • Click OK, and then click Close.

     
     
     
     
   
   
   • 
     

     If you don't see the Sign Message button, you might not have a digital ID configured to digitally sign messages, and you need to do
     the following to install a digital signature.

     
     
     
     
     

     • On the File menu, click Options > Trust Center.

     
     

     • Under Microsoft Outlook Trust Center, click Trust Center Settings > Email Security

     
     

     • Click Import/Export to import a digital ID from a file on your computer, or click Get digital IDs to find a list of services that
       issue digital IDs for your use.

     
     
     
     
   
   
   
   



2. Compose your message, and then send it.

Digitally sign all messages

1. On the File tab, click Options >Trust Center.




2. Under Microsoft Outlook Trust Center, click Trust Center Settings.




3. On the Email Security tab, under Encrypted Mail, select the Add digital signature to outgoing messages check box.



4. 
   

   If available, you can select one of the following options:

   
   
   
   
   

   • If you want recipients who don't have S/MIME security to be able to read the message, select the Send clear text signed message when
     sending signed messages check box. By default, this check box is
     selected.

   
   

   • To verify that your digitally signed message was received unaltered by the intended recipients, select the Request S/MIME
     receipt for all S/MIME signed messages check box. You can request
     notification telling you who opened the message and when it was
     opened, When you send a message that uses an S/MIME return receipt
     request, this verification information is returned as a message sent
     to your Inbox.

   
   
   
   



5. To change additional settings, such as choosing between multiple certificates to use, click Settings.




6. Click OK on each open dialog box.