Sharepoint - Is it possible to deactivate/remove/uninstall a default client side webpart?

I dont think so there is any way to remove those OOTB shipped client side web parts. But as a workaround you can inject custom CSS to your page using SPFx Extensions - Application Customizer (still in preview so workaround doesnt apply to production tenant) to hide the element from the page.

The yammer webpart render as button having following code. Use the css selector to hide it.

<button class="CanvasToolboxItem CanvasToolboxItemFloat" title="Display Yammer conversations." data-automation-id="YammerEmbedWebPartToolboxItem" tabindex="0" style="">
    <i role="presentation" aria-hidden="true" data-icon-name="YammerLogo" class="ms-Icon css-liugll CanvasToolboxItem-icon"></i>
    <div data-automation-id="less-text" title="Yammer feed" class="CanvasToolboxItem-title">Yammer feed</div>
</button>

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