An agonist works with the receptor: a substance that initiates a physiological response when combined with a receptor; in the case of salbutamol (or albutarol in the US) they activate the beta-2 receptor on the muscles surrounding the airways. They are also known as sympathomimetics, as they imitate β2 adrenergics.
A blocker (or antagonist) is one that that blocks the action of endogenous catecholamines; for that reason, they are also called sympatholytic: antagonistic to or inhibiting the transmission of nerve impulses in the sympathetic nervous system.
It appears you have experienced a simple confusion in terms. Salbutamol/albuterol is definitely a sympathomimetic, used extensively on asthma and COPD.
Pharmacology and Therapeutics of Bronchodilators