You can restrict certain options in Zoiper as described here.
However, since you are using Zoiper Classic the restriction list is a slightly different:
options_sip_accounts - no SIP accounts can be added, edited, deleted and displayed in the Options window.
options_iax_accounts - no IAX accounts can be added, edited, deleted and displayed in the Options window.
options_sip_new_account - no SIP accounts can be added.
options_iax_new_account - no IAX accounts can be added.
options_audio_general - no General Audio Options will be displayed in the Options window.
options_audio_devices - no Audio Devices Options will be displayed in the Options window.
options_audio_codecs - no Audio Codecs Options will be displayed in the Options window.
options_general - no General Options will be displayed in the Options window.
options_call_events - no General Call Events Options will be displayed in the Options window.
options_integration - no General Integration/Automation Options will be displayed in the Options window.
options_provision - no Provision Options will be displayed in the Options window.
options_sip - no SIP Options will be displayed in the Options window.
options_iax - no IAX Options will be displayed in the Options window.
options_rtp - no RTP Options will be displayed in the Options window.
options_stun - no Default STUN Options will be displayed in the Options window.
options_diagnostics - no Diagnostics Options will be displayed in the Options window.
options_network - no Network Options will be displayed in the Options window.
options_fax - no Fax Options will be displayed in the Options window.
options_sip_accounts_overview – no SIP accounts overview will be displayed in the Options window.
options_iax_accounts_overview – no IAX accounts overview will be displayed in the Options window.
hold - the Hold button in the main window will be disabled.
forwarding_autoanswer - the Forwarding/Auto-answer button in the main window will be disabled.
transfer - the Transfer button in the main window will be disabled.
addressbook - the Addressbook button in the main window will be disabled.
history - the History button in the main window will be disabled.
conference - the Conference button in the main window will be disabled.
check_voicemail - the Voicemail button in the main window will be disabled.
options - the Options button in the main window will be disabled.
dtmf - the Show DTMF Panel button in the main window will be disabled.
quick_dial - the Show Quick Dial Panel button in the main window will be disabled.
account_selection - the Account selection combo box in the main window will be disabled.
register - the Register button in the main window will be disabled.
mute - the Mute Input and Output buttons in the main window will be disabled.
volume - the Input and Output Volume sliders in the main window will be disabled.
phone_to_dial - the ability to enter phone number in the “Phone to dial” filed will be disabled.
dial – the ability to dial using the Dial button, history and address book will be disabled.
hangup – the ability to hangup a call will be disabled.
dnd - the DND button will be disabled.
The tag should be put on the same level as <general> tag for example Here is an example <restrictions>hangup,hold</restrictions>
You can put it between
</general>
<general_additional>