There are several factors due to which an applicant would become ineligible to enter Canada.

While outside Canada

Applicants are not allowed to come to Canada if had been involved in criminal activity, in human rights violations or in organized crime. You can also be inadmissible for security, health or financial reasons.

Applicants who have been committed or been convicted of a criminal offence would also be inadmissible to obtain a visa to enter Canada.

While in Canada

Whilst in the country on any of the above tourist visas you are not permitted to undertake employment. 

If you wish to work during your time in the country, you will need an accompanying Canadian work permit. You may study so long as the course is short term and will not exceed the six months expiry date.