No, you cannot delete a marketplace from your SelluSeller account. However, you can disconnect it from your account at any time.
Once you disconnect the Marketplace your products still be there on respective Marketplaces however the data will not flow from SelluSeller to Marketplaces and vise versa post disconnection.
Hope this answers your question!
Click here to know how you can disconnect a marketplace from your SelluSeller account.