1) It is possible to disable the warning by clicking "Estimation control.." and then ticking "suppress numeric warnings". This only removes the messages however so your model will not converge any better than if you told it to continue estimating every time you get the message.
2) If the model runs non-stop it might indicate that that either it isn't specified correctly or that there is something wrong with the data. A common problem is that the data isn't sorted correctly (see http://www.bristol.ac.uk/cmm/software/support/support-faqs/worksheets.html#order
). To check this open the hierarchy view (Model->Hierarchy View) and check that the number of units reported at each level matches what you expect.
For further information on sources of error when running R(IGLS) models see http://www.bristol.ac.uk/cmm/software/support/support-faqs/errors.html